Tracphone minutes roll over as long as you keep the service paid. In other words, you buy a card for X minutes and X days, the minutes and months are added to your service. If you run out of either you have to add more, but the numbers just add, the minutes don't ever reset to 0. (I'm probably not describing it well.)

I use trac for my personal cell because I use very few minutes, and that makes it cheap for me.