Fuel stabilizer on a fresh tank of premium gas. A full tank allows for less evaporation in my experience. Other than that, putting it on blocks under the suspension not the frame will help out the tires and keep the suspension pieces close to where they want to be. You don't want to let the suspension droop for extended periods as it will stretch the rubber bits. Of course a good bath inside and out helps with a couple pieces of regular cheapo (not mesquite or quick start) charcoal in a brown bag under the seat will help keep any mildew smells away. Cover it if you can and don't let the kiddos stack crap on it.
+1, also take the battery out and put a trickle-charger on it. I had 2 Mercedes batteries go bad within 2 years due to leaving the batteries connected as I left them in storage.