Most sites won't accept US credit cards. However there are some VISA
/MC gift cards (like the All Access brand cards) that usually DO work at most sites, a lot of folks pick those up at a convenience store, load them with money, and deposit from that. It costs a few bucks extra, but if you deposit enough up front and manage your bankroll, the one-time fee isn't bad.
SportsBook.com, which runs a Cake skin, does accept most US credit cards at this time. They took mine, which didn't work anywhere else. Their poker room isn't too bad, I've done pretty well there, but it's a little buggy and definitely not up to the quality of Stars. You can however use your account for other gambling (sports, etc.).
Otherwise most US players either use snail mail or bank draft to fund or withdraw their accounts. I've used eCheck on PokerStars
and my credit card on SportsBook. So far I haven't funded my FullTilt account but will likely use eCheck or the equivalent if I do.