I have recently ran a few contest with some success and here is what I did. For each contest, members were allowed to enter 3 times, once through the site, once through facebook and once through twitter.
All they had to do to enter through the site was register and post a reply to the contest thread.
I used shortstack.com to setup a fan gate on Facebook.com so they have to like my fan page before they could enter the contest.
I used rowfeeder.com and made them tweet a specific phrase with a certain #HashTag to enter the contest.
All in all, this turned out to be pretty successful. I am going to be running a number of additional contests in the next couple of months. Most of my prizes maxed out at $25. The killer was paying for shipping to Greece or the Philippines, LOL.
This type of contest was a random drawing between all the entries. (Facebook, Twitter and the site)
The other contest I did was guess the stats. If you watch american football, I asked all my members to guess the 3 stats for the QB for a specific game. The person with the closest guess, won the prize. Next time I am going to give them an "extra" entry for liking my facebook fan page.