Primarily, rinse it so dirt does not hold moisture on it. .
Secondarily, if rust emerges, clean it with wire brush then naval jelly or some such, and then treat it with POR 15 or Rust Bullet, which scavenges the oxygen out of the residue of the corrosion and leaves a solid finish. When dry topcoat that with undercoating.
Be sure the battery is properly vented or else the acid fumes *will* eat a hole in the floor under the back seat in 12-18 months (some list members found that one out the hard way).
Finally, don't get the insides (too) wet. Cabin moisture, unchecked, will combine with the dirt and will eat it from the inside out. If you have sunroof, then you have sunroof drains and you need to be sure they are connected and they are not clogged. There's other posts about that on this site.
