Traxxas has some new RCs featuring waterproof brushless systems but you can also do your own waterproofing on almost any hobby-grade RCs. With toy-grade it might be more difficult because you generally don't have easy access to the internal parts -- but if you don't mind tearing it up a bit you could open it up and do some waterproofing of the toy RCs too.
WD-40 If you decide to do some muddin' without special preparations, after a good cleanup squirt some WD-40 in the gear box and back of motor if possible. Let it dry for about a 24 hours before running to give it time to dry out. It won't prevent all damage but it can help in a pinch.
Tape and Balloons Electrical tape and balloons are two methods of protecting delicate internal parts. Wrap tape around exposed wires and connections. Use a balloon to protect the receiver pack. Balloons provide great protection of internal parts from dirt and moisture. But don't use a balloon on your ESC.
Silcone Among other Snow Proofing Tips (which can apply to mud and water as well), Misbehavin' RC suggests using silicone sealant. on servos that aren't already designated as waterproof.
Receivers and Servos At the HobbyTalk forums, the Hankster outlines waterproofing and general protection tips specifically for your Nitro RC. He's got detailed instructions and photos on how to cover your receiver and servos with balloons.
Clean Up No matter what the weather or how well you've waterproofed your RC, you always need to do proper after-run maintenance when you're done.