1. Turn your speaker volume down REALLY LOW and see if you can still hear all the syllables of the lead vocal clearly. Then see if you can still hear each individual instrument. The fresh perspective can be eye-opening (or is that ear-opening or mind-blowing??) for me.
2. Also with low speaker volume, play the song while checking e-mails or reading some posts on this forum. When I am only "half listening" I notice things I missed when paying full attention.
3. Although I use only sampled or synth bass instruments, I still find I like the result of adding compression to the bass track to increase its sustain level on many songs.
4. This one is more production than mixing: I think I read years ago that Michael Jackson would have several tracks where he just whispered along with his lead vocal, then they mixed it in for a cool "subliminal effect". I've found that if I make one good "whisper track" and edit it to tightly match my lead vocal's timing, I can get that cool "breathy vocal sound" without having to sing that way, which can be very hard on the vocal chords. Then I don't need to boost treble on the lead vocal, which causes problems with s's and t's and some other consonants. When editing the whisper track, I often just erase the s's because they are not needed. I end up editing the volume of the whisper track a lot to get the blend I want, perhaps because of my limited singing skills

Anyway, what are your favorite mixing (or production) tips?
Doug