My cell phone use is pretty minimal so I went with a pre-pay (tracfone). They allow you to get a years worth of service rather than having to renew every month or 90 days. I think it's about $99 to get a year and around 1,000 minutes (my phone came with a double min for life special).
For me this is more than enough. In fact if your use is small like me, you usually end up having to put on more service time and more min before you run out of min.... That's the only thing that I don't care for about the system. Minutes and time can't be purchased separately, but it still is a whole lot less than getting a phone with a monthly plan.