I had the same problem with my 92 SSEI. Being cheap, I went to my local Radio Shack and bought a five pack of 1/2" magnets ($1.89): http://www.radioshack.com/product/in...entPage=search
Get some gel style super glue and some kind of degreaser or alchohol. Clean the inside outer edge of your gas door. Also clean the inside area next to your filler tube.
You need just two of the 1/2" magnets. Clean them and position them next to the filler tube (body side) and close the gas door to see how well they fit and close. On my car, the magnets (from top to botton...are separated by a couple of inches). Once you get the position right, just apply a little gel super glue and stick them on (body side).
The magnets are just the right thickness so the door closes flush with the body. The only drawback is when you go to fill up. The door will flap around since the spring is broken. Still, it'* a small price to pay to keep the gas door shut while driving down the road. Going through a car wash or flying down the road at 80...no problems!
After five years, the magnets are still stuck on...
92 SSEI 140,000 miles