Originally Posted by
polonius
I bought an AT&T sim last time I was in the USA. The only way to keep it from being cancelled in a month is to buy 100 USD worth of credit, then it it good for a year. As long as you put another 100 USD on it within the year, it will not expire. This works for me, as I visit the USA about once a year and use about 100 USD when I do. May not work for everyone.
Or get a T-Mobile $100 voucher which gives you a year and you can just add $10 for the next year to keep the account active.