Sta-bil fuel stabilizer, and leave the tank dead full to prevent rust. Always try to burn the tank empty to get rid of old fuel, and store with fresh fuel and Sta-bil.

I've got a great uncle who dumps a pint of dexron in his fuel tank once a year, and he has never rebuilt the motor in his '70 300SEL, and she's got over 300k miles on her without a tick or a rattle from the engine. He claims the high detergent action really cleans the brown varnish off all the metal parts, like lifters and piston ring grooves, and the mineral oil softens and swells the seals to compensate for wear and aging. This is a guy who spit shines his garage floor, and I guarantee he has no leaks, either.