If your plug‑in synths sounds feel a bit cold and clinical, there are plenty of ways to inject some life...
Many analogue synths seem to offer living, breathing sounds that change slightly with every note you play. These appealing quirks may be due to the circuitry warming up, resulting in oscillators changing their tuning slightly over time, or perhaps the response of their filters, amplifiers, and so on. Digital synths give you a more powerful means of control, and enable you to fire up a project and know that you'll get the same sound as last time — but there's a trade-off, because the precision can make things feel predictable, and 'sterile' or 'boring'. If you don't own any analogue synths, though, don't have the space for them, or simply want the convenience of plug‑ins, how can you get that bit more bite, movement, and je ne sais quoi from your digital synths?
A Breath Of Air
A quick way to inject that extra something into a bland sound is to pass it through an amplifier/loudspeaker combination, and record it, complete with a bit of real, moving air, using a mic. You needn't use expensive gear, either: while people expect a guitar to sound a certain way, you're much less constrained by convention with synths, so a practice amp, multimedia grot‑box or boom‑box may do the trick just as well as a boutique guitar amp. Re‑amping can add distortion, warmth, compression and 'room tone' to many sounds, softening synthesized drums, and adding an edge to bass parts, or helping super‑clean pads sit better in a mix.
Amp Modelling
Miked up or modelled, guitar amps and speakers can add bags of character to 'synthetic' sounds, so don't just reserve them for the guitars. 
There are plenty of hardware and software devices that simulate amp/speaker cabinet combinations, too. Borrow a Line 6 Pod or Behringer V‑Amp from a guitar‑owning friend, for instance, or on the plug‑in side try something like Softube's Vintage Amp Room, Native Instruments' Guitar Rig or IK Multimedia's Amplitube. Amp modelling can work particularly well with lead synths, since you can quickly wade through a variety of treatments to find the most 'organic' sound.
Physical Movement
If you do opt for the old‑school method of miking up a speaker, there's no reason to limit yourself to a single, fixed mic: try several simultaneously, placed at different distances from your loudspeakers. Record each to a separate channel, and use automation to move between them, or, if you can find one, try recording a rotating Leslie cabinet instead, using a couple of mics to capture stereo movement. Even dangling a mic by its cable and swaying it backwards and forwards or in a circle can provide intriguing results.
Low-harmonic Distortion
If your synths sound too clean, some carefully selected low-harmonic distortion can add body, richness and thickness. It's something that works particularly well on drum machines and bass lines, but you can warm up pad and lead sounds too, while preventing them dominating the mix. There are loads of plug‑ins offering obvious distortion, but many may be too 'in your face' for harmonic enhancement. It's well worth messing about with different models, but you could also try experimenting with preamps...
Tubes & Transformers
Valve or tube gear (above) can impart warmth and distortion, but audio transformers (below) are also responsible for some of the classic 'warm' character of old consoles and outboard gear.
There are plenty of tube preamps that are specifically designed to offer extra 'warmth' and 'character', and these don't have to be bank‑breakingly expensive — although if you'd like to splash the cash there's plenty of esoteric tube‑based hardware such as Cranesong's HEDD (Harmonically Enhanced Digital Device) offering triode, pentode, and tape enhancement, and Thermionic Culture's versatile Culture Vulture.
Many characteristics that some people assume are 'tube' enhancements are actually provided not by valves at all, but by audio transformers. It is often these that provide extra warmth on vintage outboard gear and consoles, due to the non‑linear core saturation that occurs when they are driven hard. Useful with almost any synth sound, you can try out elderly gear (or more modern preamps based on such designs) for its transformer effects. Even budget DI boxes and ultra‑cheap audio ground-loop isolators might add something pleasant. Again, software modelling is getting better, and you could opt to use a purpose‑designed plug‑in such as URS Saturation.
Distortion As An Effect
More obvious distortion can introduce an edge on lead, bass and drum‑synth parts. There are lots of ways to add it, including preamps, channel strips, effects pedals and software plug‑ins. A couple of the most versatile plug‑ins I've used are Izotope's Trash and Stillwell Audio's BadBussMojo, both of which can also be used in a more subtle fashion if required. On the hardware side, running an analogue mixer channel into overload can be fruitful, as can sending the signal through guitar amps. Perhaps a subtler approach is to use a distorted version in parallel with the untreated sound, so that you get a clean underbody with a dirty crust on top, to enhance the detail in the sound without drowning it.
Effects & Stomp‑boxes
Another fruitful area for the synth experimenter is hardware effect boxes, which offer a huge range of effects, still unmatched in some respects by software emulations, particularly in the case of phasers, vocoders, wah‑wah and the like. Old classics from makes such as Electro‑Harmonix, Boss and MXR are still very popular, while more expensive rackmount devices cover a huge amount of sonic ground. Modulation-based effects are particularly good for fleshing out pad sounds. Synths put out higher levels than most guitars, so you'll need to keep their output levels lower than normal if you want to avoid distortion with stomp boxes. That said, you're unlikely to damage their circuitry even if it sounds overloaded.
The Equaliser
Adding some kinks to a synth's frequency response may make all the difference. Try rolling off the top end for more mellowness, using a low‑pass filter (perhaps with a resonant peak for extra character) or high‑shelf filter, or adding a low‑frequency 'hump' to warm them up. Many EQ designs offer a selection of shelving and peaking responses for different characters, and once again both software plug‑ins and hardware equivalents can be pressed into service. Anything goes here: esoteric hardware from legendary names such as Manley, Massenburg, Neve, and Pultec remains particularly desirable, but as we're after flavour here there are plenty of more affordable options. Again, some such units will add more than simple EQ, with circuits using valves, transformers and other goodies. There are some great EQ plug‑ins that model this sort of hardware and, as with distortion, it's well worth trying a few different ones out to get the right sound.
The Reel Thing
A good quarter‑inch stereo tape machine can work wonders. Plug‑ins (such as Bootsie's Ferric) might not recreate all of tape's idiosyncrasies, but are still a great way to add warmth and saturation.
Bootsie's Ferric plug-in.Running audio onto analogue tape results in extra warmth, due to the 'head bump' EQ, a high-frequency peak and roll‑off whose frequency and amount are dependent both on tape speed and on magnetic saturation (compression) as you drive the tape even harder. For that classic 'tape' sound, you'll ideally need a reasonable quality second‑hand tape machine — something like a Revox B77, which offers stereo recording onto quarter‑inch tape, should do the trick. Remember that to properly add its magic, it will also need to be well set up and have suitable tape available. Some people even suggest pressing old compact cassette decks into service: although these will seriously restrict your dynamic range, as well as adding plenty of background hiss, the aim isn't always to get a clean sound.
If the idea of setting up and maintaining tape machines doesn't float your boat, there are plenty of emulations around. The best are still in hardware, such as the Anamod ATS1, Rupert Neve Designs' Portico 5042 and the Empirical Labs Fatso — all of which will leave a generously sized hole in your wallet. Software hasn't quite 'got there' yet, in terms of accurately modelling the behaviour of tape, but many plug‑ins nonetheless get near enough to do a good job of warming up synth sounds, and they're well worth a try.
Acting On Impulse
Many mics, amps and tape machines have bumps and ripples in their frequency responses that at least partly define their character, and hardware such as Focusrite's Liquid Mix can closely emulate the character of various EQs and compressors using dynamic convolution — so if you have a unit like this, give it a go. If you don't, using any convolution reverb plug‑in with a short impulse response still comes a reasonable second. Noisevault (www.noisevault.com) offers a good selection of cabinet and preamp IRs, and there are two zipped Beamsonic collections at http://noox.sitesled.com that contain a handy selection of microphones, tape machines, and rack effects, and a good range of guitar amp/cabinets. Body resonances of acoustic instruments can also be captured and added to your synth sounds.
Compression & Limiting
Synths can sometimes have unpredictable output levels (especially when filter resonance is turned up high), so a little compression can prevent unwelcome surprises. Heavy compression can also be a great way to add punch and increase character, particularly on lead, bass and drum sounds. Remember to keep the compressor attack time high enough to retain the initial attack transient of your sound, and that, as with distortion, you can also mix in a little highly compressed signal with the untreated version to taste.
Layering
If a solitary synth lacks something, why not add another alongside, playing the same line but with different patch settings? When most modern computers can run dozens — or even hundreds — of soft synths, this can be as simple as duplicating an existing MIDI track and pointing it at another synth. The dance fraternity have been layering sampled drum sounds like this for years to achieve the right mix of depth, bottom end, thwack, snap, and suchlike, and this technique can also be pressed into service for creating rich, evolving bass sounds and pads. You could pan one source left and another right, EQ them differently, or apply different effects to each. Just be careful not to overdo things such that you obscure the focus of the mix.
Lo‑Fi Treatments
De La Mancha's Imperfection is designed to make it easy to add a bit of grit and dirt.
Apart from all the techniques presented here, there are also plug‑ins dedicated to reintroducing grit, vinyl crackles and other background noises for extra texture. Examples include De La Mancha's Imperfection and OtiumFX's Sonitex. They won't be to everyone's taste, but that's part of the joy of music making. Also bear in mind that you don't have to use analogue gear or analogue models to create a warm character: early digital devices were full of idiosyncracies born of imperfect design. Try running your synths through a bit‑crusher, for example, and you'll see what I mean.
Final Thoughts
Some of these suggestions will be routine, but others, hopefully, totally out of the box (some quite literally!). Either way, I hope they'll inspire you to explore new ways to inject extra life into your synthesized creations. At the end of the day, you just have to use your ears and remember the old mantra: if it sounds good — it is good.
Humanisation & Timing Tweaks
Sometimes it's not the sound of a synth that makes it lifeless, but heavy quantisation. Even tight‑sounding drums and bass lines invariably work around the beat, pulling it ahead to add urgency or lagging behind to make it more laid back. If you have a stiff and stilted MIDI performance, try adding randomness to the timing and velocity of each note, or randomising note lengths to make filter sweeps alter from note to note. There are also specialised MIDI 'humanise' plug‑ins (PC users could try Frank's Humanize at www.midi‑plugins.de/mplug/mplug‑hum.html) that not only alter note start times, duration and velocities according to human nuances, but can also add small amounts of randomised pitch-bend data. If you do need to quantise, choose a type that allows you to pull notes gradually towards perfection (in Cubase this is called Iterative quantise) and stop when it feels tight enough. Of course, you could choose not to quantise your performance at all: just treat the synth like any other instrument, practising until you can play the part as you want it and selecting (or compiling) the best take.
Programmed Movement
One of the quickest ways to make a synth sound more interesting is to vary parameters, either in real time or using an LFO. As well as obvious things like filter cutoff frequency and resonance, try automating parameters such as envelope attack time or filter envelope amount. On modelled synths, changing almost any parameter can bring note‑to‑note variations that characterise acoustic instruments. For a more human touch, try recording several passes of automation on different parameters and, if available, use MIDI controllers with knobs and faders, or pedals for swells and keyboard aftertouch.