I usually clean it out when I wash my Jeep; either a regular hose or power washing station will do. If you do it regular enough it wont get dirty enough to require constant re-coating to keep it looking spiffy.
If you haven't done it before - soak it with some degreaser, let it sit, then go through it with a power washer to get all the accumulated gunk off (go to one of those pay to use stations along the main roads). Then any spots that look faded or nasty - hit them up with some semi gloss or undercoating (if you live in a salty area). After that; just keep rinsing as I said earlier - when you're rinsing the body, just crouch down on all corners and hit EVERYTHING (oil pan, tcase, tranny, axles, frame rails, etc.). I've been doing this for over a year now and it turned my faded cruddy looking frame and drivetrain into something that looks like it rolled off the dealer a few months ago.
If you went through a mud hole (EVIL!) and it's caked on there; put an oscilatting sprinkler under it and let it run off for an hour or so, then do the fine detail work with the hose/washer.
