Find out what the best carrier in the locations you're going to be is. (Use the coverage maps to start.) Then get a phone on that carrier and get an account. Or, if your phone is GSM and unlocked, just get a SIM for that carrier. (If it's a GSM phone you can't use it on Verizon or Sprint or any of their mvnos - they're CDMA carriers, It's a different technology. And if it's a CDMA phone that wasn't originally on Sprint, Sprint won't allow it on their network.)