I pick up a junkyard spare, clean it out and refresh the brushes if needed and take it with me. If you do any mudding, an extra starter is just about a must. Trying to change a starter in the mud is hard enough, trying to remove, clean and reinstall what you've got, is more than twice as difficult.
A good tip if you do much mudding, is try and get as much extra cable down low as possible (undo the straps and reroute the wires some). The electrical connections can be hard to get to with the starter still bolted up, but get much easier if you can get the starter unbolted and down low enough. A little extra cable helps.