Here's my solution! I turn each nail polish upside down in my rack for about an hour when I'm cleaning or whenever. I do this about once a month, just so the nail polishes don't get all separated. I've found that I can't turn every one upside down at once because they will end up falling off of the rack. I just do every other one so that the one beside it is able to stabilize the polish upside down. It's actually worked really well for me!
Do you have any tricks to keep your nail polishes nice and mixed?