Use Promotions

Make sure people get something in return for liking your website with a promotional offer that they can easily redeem.  The more lucrative the offer is, the more sharing will happen between Facebook friends.  Fans of your site will want exclusive products, discounted prices, or to be notified early about upcoming events or specials that you are running.  Try to make the promotions give each fan the feel that they are special, not just a standard shopper.
Create a special landing page on your site that targets Facebook visitors.  Reinforce your promotions on the landing page and link your Facebook marketing to promote the landing page.  Be sure to put the social networking links on this page to encourage your fans to share this page.