The best thing I can say is: learn about sound. Learn about frequencies, which ones sound like what and do what to the overall mix. Whether or not to use a low or high pass filter on something. Fiddle with EQs for hours. And more importantly, get criticism. It's the easiest way to learn what sound to look out for, and such. And most importantly of all: Practice. Mix till your ears bleed. (from the time spent listening to the same song, not due to the terrible mix :P)
Mixing is one of my strong suits, but most of it is just being able to hear the song, and make the adjustments that I want, to be able to make the necessary changes and just know what needs to be done.