The more music I listen to lately the more I'm surprised by how much of it is basically mixed in mono. Sure there might be some little things in the highs whizzing back and forth to generate a little interest but that gets old pretty fast and the real meat and potatoes is still pretty much solidly, unmovingly, up the middle. In the forums you find all kinds of people saying mono doesn't matter any more. Reality would seem to indicate otherwise.
For that reason I'm always looking to find ways to create space and if possible, motion in my mixes. That doesn't mean wild panning automation. There's a lot of little things you can do. Things that you have to do. Because once the ear gets used to something, it disappears.
One of my favorite tricks, both live and in the studio, is to mic a source twice, hard pan, and EQ each side differently. You can use two of the same mic or two different ones, it's half of one and six dozen of the other. Or something. That's about the most basic step you can take to widen the image of something that's basically parked at center.
In a DAW, if I don't have two mics on a source, or I'm stuck with a single, mono track I'll just duplicate it and start in on the EQ. My favorite EQ plugin has a neat option that allows you to flip the curves upside down. I never had a use for it until I spotted it while working through this trick. I put the EQ on the left instance and cut some 400 Hz out of a guitar, then boosted 2.5 kHz. I copied the plugin instance to the other track and checked the reverse box. Boom! Evil twin guitars. They're playing the exact same note at the exact same instant but because of the panning and EQ when the music goes lower it sounds like it's coming a little more from the left and when he goes up the neck it moves to the right. You can accentuate the motion even further by using some subtle compression after the EQ.
Don't just sit there Brethren of the Knob and Fader. What are your favorite tricks for getting space and motion in a mix?