I have to post my two favorites - both from the same gas station/minimart about a year apart. Both were very nice signs.

1. No gas will be sold to anyone in glass containers.

and, at the minimart counter
2. You MUST tell the cashier if you've got gas.

I was laughing so hard as I bought my soda and tried to pay for the tank I just filled at the minimart counter that the cashier took pity on me and said,
"It's OK, I know you've got gas".
It's a little scary posting to this thread, though, I'm sure I've made a few grammatical errors that hopefully won't detract from the signs.

Edit - I didn't fill up my tank at the minimart counter, I filled it up out at the pump and was trying to pay for the gas at the counter.