One of the main problems when you're producing your own music is getting the balance of instruments right in the mix. It doesn't matter whether the sound source in question is a single multitimbral MIDI module or a multitrack tape machine, the problem is exactly the same, and after several hours, or even days, of working on the same piece of music, it's very easy to end up completely lost and with no objectivity by the time you come to do the all‑important mix.
The old rule is that you should never mix on the same day as you finish a recording, but, unfortunately, for many of us that's just not possible. In a commercial studio environment there is considerable pressure to get the mix done straight away, while in a private recording situation, impatience usually wins out over logic.
If you can find the time to play a couple of commercial recordings through your monitoring system, preferably in a similar style to the music you're trying to mix, that will help to restore a sense of perspective, but I've found that by far the most useful tactic is to listen to the mix from outside the room you're recording in. In other words, if you do your recording in your bedroom, leave the bedroom door open and go and listen from out on the landing.
This might seem an unusual strategy, but for some inadequately explained reason, it always seems to work. That hi‑hat which seemed absolutely fine when you were sitting in front of the speakers is now quite obviously far too loud, while the vocals are virtually inaudible. I can't explain why this should be, but any balance defects just seem to leap out at you.
Once you've made the necessary corrections to your instrument levels, the mix should sound right both from inside the room and from outside. I used to think I was the only person who used this method of mix evaluation, but having spoken to a number of industry professionals who do exactly the same, I feel rather better about my sanity. Try it; I think you'll find it far easier than dashing back and forth to the car stereo with dubious cassette copies!