I use this when I'm in the U.S. for more than a couple of weeks at a time, T-mobile has the most flexible pre-paid plans for data that I've found. You need to c

I use this when I'm in the U.S. for more than a couple of weeks at a time, T-mobile has the most flexible pre-paid plans for data that I've found. You need to call them or go to a store to buy more data though, their website doesn't accept foreign credit cards even though they allow you to choose your country.