GO TO A page you like (one you are already a fan of) and look at the links right under the "profile picture" at the top left of the main page. You will see one of the links says "Suggest to Friends." Click on that link.
You will see all your Facebook friends. You can now just go through your friends clicking on whichever ones you want to get an invitation to "Like" the page that you like. When you're done, click the "Send Invitations" box.