First of all, try to get the "good" non ethanol gas, otherwise it doesn't store as well. Second, use a good gas preservative, I swear by PRI-G, I have stored gas using PRI-G for over a year and had no issues when I used it.

I used to use Stabil, but the Stabil itself has a shelf life,not so with PRI-G as well as I am able to use PRI-G to restore older gas that hasn't been stored properly so that I can use it again.

Here is some literature from the company on it:

PRI-G Fuel Stabilization

PRI-G Article

As it is, use PRI-G and I try to rotate 1/2 of my stock every 6 months into the cars and then replace it, that way I am not replacing 50 gallons at a time which could break the bank!!