3. Distance

THE PROXIMITY EFFECT:
The proximity effect is a term meaning the closer you are to a source, the more low frequencies you'll hear from it. This is why your voice will sound boomy if you have the mic an inch from your nose. By the same token, the closer you have a mic to a guitar speaker or any source, the more bottom it will have.

One way to reduce some of that unwanted low end on vocals, and also reduce "plosives" (really loud P's, B's, etc.) or sibilance is to position the mic higher than the singer. Have it high so they tilt their head up to sing, but aim a bit below the mic. (For some vocalists I've recorded I just put a dummy mic up for them to aim at.) For traditional (non-screaming, non-folky, melodic) vocals, having the vocalist stand and tilt their head up to sing stretches the throat in a way that's good for breathing and singing.

There are some other phenomenon that seem kind of contradictory to the proximity effect. For instance, the closer something gets to your ear the more high frequencies you'll hear from it. Think of whispers, flies, etc. Lots of high's there. Conversely if someone is down a hallway, their voice will sound muffled. You can use this in mixing a lot. If one instrument sounds too close to you, or you want to bring something closer, mess with a high-shelf EQ.
EQ can be one of the most useful tools for assigning an instrument a place in a mix, more so than reverb.

STEP THA FUCK BACK:
As I mentioned in the post on waves, the higher the frequency of a wave, the shorter the wavelength; so really low frequencies have really long wavelengths.

Which brings me to the point: You can't hear frequencies very well if you're closer than one wavelength. Take a step back from the speakers you're mixing on every now and then and hear what your mix sounds like. Sometimes pulling the mic a foot away from a kick drum or bass amp sounds good too, as long as there aren't other noise sources nearby.