The big thing I struggled with for a long time was mud, which is frequencies that detract from the clarity of your mix in the mid-lows. Mud typically exists between 200-500 Hz. To get rid of it, on each track of my mix, I use a parametric EQ with a narrow Q (usually I set the Q around 7 oct/db) and sweep across those frequencies with it boosted until I hear something that doesn't sound too good, and then cut that frequency. I often have to broaden the Q a bit for the cut to make it work. I do that 2-3 times on each track to clear up mud. Just don't get super aggressive with it and it will help a lot.
I also do a low cut up to 175-200 Hz on pretty much every track except for the bass and the kick, and that gives a ton of breathing room for the kick and bass.
Drums is another tough one, you want them to sound "phat" (which is a whole other walkthrough itself) and it helps too to understand what frequencies each drum (snare, kick, toms, etc) needs in order to sound the way it's supposed to. This guide helped me a ton: http://www.benvesco.com/blog/mixing/2008/mix-recipes-tom-eq-and-compression/ It's for toms but there's links in the first paragraph for kick and snare too.
EDIT: I know we've all posted a ton of info, but honestly, it would probably best help us figure out how to help you if you could post an example of something of yours that you're not satisfied with.