My workplace just launched a new poker site for the U.S. market, which awards cash prizes using the sweepstakes model. They're having a beta testing period for a couple weeks during which subscriptions are free, and since there aren't very many people on the site yet it'll be extremely easy to win cash prizes. Download the client software here:
http://www.fleetstreetgames.com/downloadInstall it, run it, and register an account. Then during the beta period I believe you go to the cashier to activate a free membership. Here's the direct link:
https://www.fleetstreetgames.com/cashier/join