Despite the fact that people seem to think that mono is dead I beg to differ. I've worked on quite a few projects that the left and right channels were all but identical. Even where some panning had been done the effect wasn't that powerful. The reason for that is your ear gets used to things. The best mixes have motion in them. It's not hard to get. The most basic approach would be to just automate some panning. But there are much more subtle (and less nausea inducing) ways to add a little motion.
One of my favorites is really simple to do and really flexible. Done well it can be very subtle and effective. Let's say you've got a doubled guitar track. No wait, let's make it even a little harder. Let's say you've got just one guitar track and you wish you had doubled it to put some space in your mix. The old trick of duplicating it in the DAW and adding a little delay or even a little de-tuning to one side is as old as the hills and like anything else that doesn't move, it falls flat after a while and your ear stops hearing it. So what to do?
Take those identical guitar tracks and hard pan them. Then EQ them differently. Make one a little more bottom heavy and the other a little top heavy. Then strap a compressor on each of them and make the settings different. With both compressors idle the image will be in the center. When the sound gets a little chunkier the fat side will squash down a bit and the higher sounds on the other side will seem to poke out. Likewise a high solo will squash down a little and the bottom heavy track will be a bit more audible.
It has sort of the opposite effect. When the signal goes one way, the result is an accentuating of the opposite end of the spectrum on the other side of the mix. But don't forget that you're also getting the benefit of the compression fattening up the signal going through it. So a chunking guitar riff will get fatter but also seem to sparkle a little bit, a solo will gain some texture but also get a little body. And the beauty part is that the changing dynamics on the hard panned tracks will make it all dance just a little between the two sides.
How much it dances is based on how much you squeeze each side and how heavily you've EQ'd them. Like anything, a little goes a long way. Don't make that rookie mistake of laying on your new trick all thick and spicy. The Brethren of the Knob and Fader live by a code of tasteful application. Learn this trick and do it well and people will be excited by your mixes and not know why. You can just smile the smile of a true wizzard and know how the magic really works.