There are as many tips for mixing music as there are people who mix music, but a few of these tips are common to just about anyone who has spent any significant time mixing. Joe Gilder just put out an article about the mixing lessons that were most significant to him in 2012, and not surprisingly, some of these universal mixing tips were on there.
One of the items on his list - the mix is slave to the recording - could be the most important lesson in his list, and was interestingly, the topic of the post I wrote yesterday - A Lack Of Passion In A Performance Can’t Be Fixed By Effects, But… which states that it is far better to get good tones, levels and performances while in the recording stage that it is to try to fix them in the mixing/editing stage.
Another of Joe's tips is something I always, always, always (getting the idea?) do, and that is to test your mix on crappy speakers. In my case I usually do this in the car while driving around in traffic, where a good chunk of my audience will be listening to my music. The point of this is to ensure a portable mix, one that sounds good on any system it's played on. If your mix has flaws, these crappy speakers will often highlight them better than your "good" speakers. This is doubly important if you do your mixing and mastering in a less-than-perfect acoustic space.
For the rest of the tips on Joe's list, see his article here: