It's best to have the gas tank full (with Sta-Bil or some other fuel stabilizer) if it is to be stored for a long time. This helps to avoid getting moisture in the gas due to condensation; if there's little room for air in the tank, there's correspondingly little condensation from the bit of air that is there.
Onan recommends running the generator under load for half an hour or so monthly to keep the brushes and slip rings clean and prevent the carburetor from getting gummed up.
Other than that, there's not much to it. I recommend leaving a roof vent or two cracked open (assuming you have vent covers) to allow some air exchange and setting a few mouse traps and removing foodstuffs. It would, of course, be a very good idea to take any water-based things out, as well, so they don't freeze and possibly burst their containers and cause a mess.
