I like to use the Tamiya Weathering Sticks in Mud, Light Earth, and Sand.  They even make one for Snow.  You can find them at RCCommand.com: http://www.rccommand.com/tank_stuff.php  They're very easy to use and almost impossible to mess up, since the object is to apply messy dirt and mud.  Just smear the stick's contents on any and every surface you want all goopy.</p>