It depends what you've done already and what truck you have. Cutting a hole in the body helps, I've also been working on a sort of air scoop which will collect air at the front of the truck, accellerate it, and blow it over the heat sink.
Lower percentage nitro fuel will help, as will richening the mixture.
Also, you could buy a sun shade and a fan and sit under it drinking a cold beer...that will stop you overheating lol.
Hope the above helps though.
Mat.