I plan on switching my phone service after my contract ends when Virgin has a phone that I like. I currently have a Motorola Droid with Verzion, my phone bill for 450 minutes, 250 text messages, and data costs me about $72 per month with my corporate discount. Otherwise my bill would be around $92 per month.
Virgin Mobile is a prepaid service you where you basically buy your minutes by the month. They include free unlimited data and text messaging with your plan. Since this is a prepaid plan, you do not pay any taxes. Plans starts as low as $25 per month for 300 minutes. Being a prepaid phone service, there is no such thing as "free late night" or "free weekend" minutes. $40 gets you 1200 minutes.
As long as you still have at least 1 minute left in your monthly plan you should be able to access the data part of your plan. Should you use up all your minutes before your month is up you can either buy more minutes, or "bump up" to the next month. Meaning you start immediately restart your monthly cycle by paying $25 or $40 for a new month.
There's a $60 monthly plan that gives you unlimited minutes every month, but I would say that the $40 plan would be sufficient for most people.
Virgin Mobile uses the Sprint network so if Sprint doesn't work well in your area, then you may need to consider a different carrier.
There's also BoostMobile (owned by Sprint), but the prepaid plans are a little more expensive than Virgin Mobile. I have not taken a close look yet at what they have offer. I have time since my current contract doesn't end for another 9 months.