1 - Get a hot product to give away for free
2 - Use either punchtab.com or rafflecopter.com to run a contest
3 - Invite two or more bloggers to co-host the contest
4 - Add your fan page as an option to enter the contest. That means contestants will have to like your page to earn more points and stand a better chance to win the contest.
Sample contest that generates over 200 likes per day for organizers: Lenovo IdeaPad Z585 Laptop Giveaway