Create an offering, to qualify for the offering require they like your page.
Another technique I see used to increase fans is to create a contest, then structure the contest so that people vote for the participants. In order to vote they have to be a fan of the page.
An example of this is I saw a denture place run a pumpkin carving contest for dentists. Dentists entered their pumpkins on the page and people could vote for them by "liking" the photo, of course you have to join the group to do this.
The dentists will try to get votes by requesting their user base that they "help us win this contest, vote for us on this page". Which effectively gets contest entrants to encourage their friends to like your page.
If you have a big budget I have seen some places offering a dollar for every "like" to a charity.