Elbow grease.

The only way I've found to get keep rust away is to pressure wash it the minute you get home from any form of sloppy. If you are pressed for time it doesn't have to be an all-out wash job, but at the very least make sure you flush the undercarriage until clean water is running off your ride.

There's trucks running around here 6 years newer than mine that are blistered and rusting, so I must be doing something right...........