First, no ethanol. Gasoline with ethanol cannot be kept alive for long, no matter what chemicals you dump in it. If all you can get is gas with ethanol, remove the ethanol from solution and replace it with toluene by volume to restore octane rating. Gasoline without ethanol will stay good for months. Toluene has a very high octane rating and is a commonly used octane booster.
Seafoam has worked well for me to preserve gasoline.