I struggled with this for a while. In my limited experience, I think the key to good mixes (especially ITB) is getting the rights mics in the right place so you can only touch the EQ when you absolutely have to. Most plug ins do more harm than good IMHO, and the good plugs are easy to over-do. To me, it seems that almost every plug sucks a little dimension from the sound.
Try going really basic - use as few mics as possible. All the gating and EQ-ing this out of that, etc. could be making it harder to really hear what's happening. A pair of overheads can be all you need. Most folks would also mic the snare and bass drum, but most of your problems will go away with overheads that are in phase, placed where they get the whole kit, in a decent room, with a decent player, with a properly-tuned, decent instrument.
If any of those elements are missing, more mics will only get in the way by adding phase problems, bleed, unwanted frequency buildups, (and the need for "corrective" Eq & gating), etc., etc., etc.
As for guitars, and anything else - Really listen to the instrument in the room. Your recording may just be perfect - it might be the instrument that sounds bad.
If all else fails, the only plug I could not live without for cutting certain frequencies and balancing a mix without sucking the life out of the sound is Voxengo's Soniformer. Looks like a multi-band comp, but functions more like a dynamic EQ.
