There are few options available to US residents these days, but the ones out there DO work well. Sites like Full Tilt and PokerStars give the player to use an Instant E-Check. With this option, your funds are credited to your account immediately, and are withdrawn from your checking account.
If you do not have a checking account, or do not want to use one, you can always use a Visa Prepaid Gift Card. Be aware, that some of them don't work, as they have similar filters that your debit card might have, thus rejecting the transactions. I've personally used Visa All Access Gift Cards with success. However, a bevvy of others (such as "Green Dot" and "Vanilla") have been rejected.