The first cleaning is important, but I find not just one good cleaning, but lots of washes, as mud begins to settle in little places, the more you wash it the better it turns out. I go to the DIY bays and spray the hell out of it for what seems like 30 mins. You can spray under the hood, but don't blast it as you might knock something loose, or get water in something you don't want water in.
Another trick is finding a place to park your truck for a bit and throw one of those curved yard sprinkler things under it and keep moving it around every 5 mins or so to help knock loose some of the crap under the truck.
The best thing you can do to clean it is just to avoid mud, I used to love that as a kid, but 10 years later I will avoid the stuff at all costs, it just is such a PITA and is much more trouble than its worth.
I also look forward to the first rain after I have been wheeling, and when it rains I try and drive my truck as much as I can, because the tires will help spray the water up under the truck in the same places it slings mud on the trail.
Good luck