This stuff works great:
http://www.303products.com/techinfo/tires1.htm
That link also provides a good explanation of tire rot causes. Couple things I do always have prevented me from ever having to deal with this problem. If I am storing the trailer for an extended period of time where it will not be used, loaded or not it's up on jack stands. If the trailer is stored outside I ensure the tires are covered with a piece of tarp so that it doesn't see direct sunlight. I treat the sidewalls with 303 twice a year. It's also great on other stuff too like vinyl. Yeah a little expensive but so is replacing tires.
