I use Virgin Mobile. I have the Minute2Minute plan, which is no longer available: $15 every 90 days, unused minutes roll over.

See
http://www.virginmobileusa.com/rates/home.do
for current rates. The Pay as You Go plans sound like what the original poster would find helpful. Unused minutes roll over if you buy new minutes every 30 days (up to a max of 5000 minutes). Costs 10 cents a minute talk time for 200 minutes, 5 cents a minute if you buy a thousand minutes.

I have gone over my minutes once in however many years I've had it, and bought more over the phone. I use the cell phone infrequently, mostly for long distance calls and when I'm volunteering at events.

I'd suggest comparing Virgin to TracPhone, ATT GoPhone, and Net10. It may be that there is better coverage or a better plan for you other than Virgin, but that's a choice you get to make.