The most important single thing you can do to boost your support on Facebook (a client of mine) is to use Facebook pages.
Why Facebook pages
Facebook page status updates appear in your fans' newsfeeds, so allow you to appear where people spend 80% of their time on Facebook.
Unlike a personal Facebook profile, where you are limited to 5,000 friends, there is no upper limit on the number of fans you can have.
Install a Facebook page widget on your other websites
If you do this people who visit your other websites will see a window on your site showing some of your fans on your Facebook page. People who visit your page, who are also logged into Facebook (extremely common now), will see their friends who are already your fans endorsing you.
Obviously since a personal endorsement from a friend is extremely powerful, this is a great help in recruiting more fans.
Look to the right of this post to see an example for me.
Advertise your page in offline materials
The easiest way to promote your page in offline materials such as leaflets and advertising is to get a vanity URL such as facebook.com/rob.blackie - which is both easier to put on offline literature and better optimised for search engines such as Google.
Facebook's self service advertising can be highly targeted based on a combination of:
The quality of the targeting means that a high proportion of clicks should convert to fans - so in some circumstances it can be very valuable. I find it particularly valuable where a brand has a well defined set of fans who are already enthusiastic.
The official Facebook guide to adverts is worth a read.