An unlimited prepaid monthly plan can cost about half of a traditional unlimited plan.
Another option: Upload an electronic version of your cell phone bill to BillShrink.com (free) or MyValidas.com ($5). They'll analyze your usage and recommend more budget-friendly plans. The sites also point out where you're overpaying. For instance, you may not use your phone enough to need an unlimited plan and could save by selecting one with fewer minutes.