First of all, when it starts overheating, pop the hood and see if its actually overheating. If its too hot you will be able to tell, the air around the engine will be signifigantly warmer than normal. Check radiator and hoses for leaks and make sure your radiator is topped off. Check the radiator every few hours to see if its losing fluid. If its not, then it could be a bad thermostat. Make sure your fans are kicking in at the right times as well.