Here's a helpful list of tips for those who want to get deeply into mixing.

http://www.cakewalk.com/Support/kb/reader.aspx/2007013312

While this is more geared to aspiring professional mixing engineers working with music sent from others, tips 8, 9 and 10 apply very well to those doing work at home on their own projects.

I especially like the tip about mixing at low levels; this works wonders for me. I also step into the next room periodically to hear what it sounds like.

Then try your mixes everywhere. And always protect your ears, solid advice for all.