In my experience, the more time you can leave yourself between rendering a mix and giving
it a second listen, the better you will be able to judge it.
As mixers we take
audio apart with our brains - we can't help it. If the last thing I've been working on is
the bass, then my ears are drawn to it and I'll often convince myself it's not right in
some way. If I can leave it a couple of days I'll listen to the mix more as a whole. If
something is genuinely wrong then it will be relatively easy to notice - and will often
not be the part you'd been concentrating on last!
When you're working
professionally you often aren't allowed the luxury of putting a mix aside for a while, but
as often as possible I will try to do this. It's a rare mix that I'll listen to later and
not wish I'd done something differently.
If this is your own work, then simply
move onto something else and come back to the mix later - you'll likely be better able to
judge what you've done.
An Eagle for an Emperor, A Kestrel for a Knave.