I'm going to mention a couple I use all the time to get the ball rolling.
LCR panning
We had a huge thread on this. But basically, by panning LCR, with the notable exception of panning things where they go in a stereo field to supplement a stereo track, that totally cleaned up my mixes and saves time as well since you aren't splitting hairs.
Remarkably, there was no mention of this for the first several years I followed recording websites, starting in the dialup days (I had gone to school for recording but there wasn't enough time to go into mixing.) But there were people telling you to pan this at 1:00 o'clock and that at 10:00 o'clock. And there still are.
Another thing is to check in mono. Turn one speaker off so you don't get any timing issues.
I had a track in which I flipped the phase of one of the stereo overheads or room mics and the whole drum sound got bigger and beefier. Then I checked it in mono and the drums all but disappeared. What the hell happened?
