If gasoline, start with non-ethanol fuel if possible. You can find it in most areas, but it's usually on the expensive end.
www.pure-gas.org is a good resource for finding it. I'm keeping my own page for locations in Utah with a focus on how good RV access is to the station.
In the past I used StaBil. They quote a year for storage, where Sea Foam quotes two. I've used Sea Foam in my Jeep before and had good results, and switched to it for storage of gasoline. Run the engine for a while after adding whatever you do to let it get into the fuel system. Then exercise it periodically.