I have WAY too many gas/diesel powered tools and toys (2 ATV's, mowers, tractor, power washer, generator, weed wacker, blower, bass boat, etc). What I've found best (and has already been mentioned more than once above):
1. Sta-bil and Seafoam added to each 5 gal can of gas (put additives in empty can, then take to gas station for fill-up).
2. Monthly running of equipment. I think this is VERY important for a generator that you may have to depend on with little/no advance warning. My gen is battery start, so the monthly running keeps the battery in good shape without having to put it on a trickle charger.
3. Any fuel remaining after 3 months goes into the truck (rarely happens for me since I use power equipment year-round here in the deep south).