The Coolest Commuter, Nick James of BikeCommuters.com posted a great cooling down article that he uses after riding his fixed gear bike through the humid streets of NYC. Check out what he has to say since you could do the same after a trail ride.
The first is keeping several bottles of tap water in my fridge. When I get home from a ride, the first thing I do is gulp down some cold water. This helps your body cool itself from the inside out.
The next step, and this is crucial on really hot days, is to fill the tub with cold water, and just soak in it for ten minutes or so. Nothing drops your body temperature faster than being immersed in cold water. Cold showers do nothing; you need to soak in it. Let your body heat radiate out into the water, and when you get out, your body will have stopped sweating.