rust is oxidized iron or steel, which is what jeeps are made from. oxidation is caused by oxygen in the atmosphere in water, which jeeps like to play in. dirt and mud that gets kicked up into your framerails and other sundry places will eventually dry out, BUT the next time it's humid or soaking, that water will be held on your frame for much longer than necessary, inviting rust to come in and camp.
if you were absolutely anal about it, you could have your frame galvanized and painted, and this would protect it, and then of course, you'd have to be pressure washing it all the time.
best thing to do, like us normal folks, clean it, but don't go nuts. make sure all your zerks get a few pumps, brake and fuel lines are especially susceptible to rust, so watch those areas, and for god sake clean the mud from the top of the skid plates now and then, or it'll weigh 50 pounds if you ever have to remove it.