I did a lot of research on this very topic recently, and even purchased some products that claim to help (and they didn't help- the rust came back in days). However, I realized the only thing you can do to stop rust is to cut it out and weld in new material.

I did this, and it was surprisingly easy. I used a hand-held grinder with cutting wheel attached to cut out the metal, and welded in new metal with a MIG welder. Then I grounded down the welds and put a layer of bondo, sanded with 200, and painted. Looks great and the rust won't come back. If it is in a highly visible area it will take many passes with the bondo and 200 (or higher) sandpaper to get it perfect. If you use electric welding, be sure to follow proper procudures to safeguard your car's electronic components and battery.

If you don't weld go to a local university and look for a student who knows how- it is way too expensive to have pro's do this for you. My total cost: $20 for paint, bondo, and metal.

Good luck,
Joe