Peter Crowl,
Along with what Accumulator said. For a DIY, undercoating is the way to go. But like Accumulator said, it can be a pain to clean later if you get the wrong type. I've used 2 different types, and here's my take on them.
Some of the undercoating in spray cans you can get are asphalt based. These are messy, and never seem to fully dry. I believe this is the type Accumulator is referencing. I've used this type first, and it does not last. It also reeks of a tar smell for days after application and looks brownish. This is on top of the fact it just doesn't really dry all the way and will wash off. I avoid this type now.
The second type is what I have on my wheel wells now. It is a rubberized undercoating/sound deadener. The can I got from Auto Zone was blue with white writing. It says on the front that it is a rubberized undercoating and sound deadener. This stuff dries fully within an hour or two, and leaves a nice, thick black finish (like OEM, atleast on my Nissan). I wash my wheel wells every car wash with car soap and a soft bristled brush, and have no problems with it coming off. I have also used it to touch up scrapes in my wife's truck bed with a spray in bed liner, and it matches almost perfectly. An added benefit to this type is the sound deadening with little rocks/gravel flinging in the wheel wells.
If you do undercoat your wheel wells, take common sense precautions. Don't do it on a windy day. Tape and mask off EVERYTHING you don't want sprayed. Clean the surface to be sprayed first. And remember that undercoating sprays much thicker than spray paint. You might want to spray a scrap first to get the feel for the texture you'll want.
Hope this helps.
Dave