There are a lot of things that can be done post production, and i think everyone has their own favorite tricks which help make up their style. I see tips for instruments, so I will share what I do with vocals.
I tend to do a blend of motown + modern on vocals. Unlike guitar, which is often panned out, vocals end up sounding odd if doubletracked. Not bad, but odd. So, once the vocal is tracked and comped, I will work with a copy of the vocals, rather than a double track.
The next thing I will do, is apply a motown style exciting compressor chain to the first copy. Boost 8khz, cut around 250hz and put a 10 - 1 compression on it. That in itself sounds really bad, but it adds a *lot* of clarity and presence to the vocal. This track gets pulled way down, and is brought up to add that presence and clarity to the vocals in the mix as needed.
With those two tracks, I should be able to get the vocal to sit in the mix nicely without having to go overboard with compression or eq on the main vocal.
Now, if I want to fatten up the vocals, I will copy the original again, add a 2 ms delay, do a couple cent pitch shift, and pan it slightly. Again, pull it down, and bring it up in the mix. Want fatter? do same again, but pan to other side, and slightly change delay and pitch shift.
Excluding the compressed mix, send the panned/deleayed/pitch shifted to a subgroup for any reverbs and delays. As far as compression, I tend to put that on the subgroup. EQ can go either way, depending on desired effect.
That motown compression trick can be applied to any instrument or track though, just change the EQ points accordingly. Heavy compressed punch and clarity, without sounding over compressed.