I keep my lotls in the coldest room of my home, away from any windows or lights that put off any heat. I keep the ceiling fan on in that room, and hardly ever turn on their tank lights. I live in Texas where we have extremely hot summers and the water from tap is warm enough to take a bath in, so I fill 5 gallon water jugs ahead of time and keep them in a closet so they cool down in time to do water changes.
Aquarium chillers are probably the best, most reliable method to keep your tank cool in my opinion. They can be rather expensive though. For a quick fix you might try mounting an aquarium fan on your tank and pointing it towards the surface of your water. I've seen many people try this method with great results.