Add airstones to the tank - warmer water can't hold as much oxygen, so you need to help as much as you can.
Removing the cover and running a fan over the surface, or multiple fans for large tanks, will help cool the water a bit.
If this doesn't cut it, you can always ice the tank - that is throw in a frozen bottle of water, or a bag of ice cubes. They'll need to be replaced regularly of course, and start small so that it doen't end up tipping to far the other way.