I have 5 sites that can be tied together by a common theme. What is the best way to link them? Right now I have links only in the content. Should I also have links to all of them at the top or bottom of the pages?
Be sure they have different c-class IP's, and in-content links are very good, so keep on what your doing i say, but header/footer links won't harm.
Well I'll presume all your sites are on the same server, using the same hosting account. Without getting to complex, you don't want to do reciprical links or anything that will make it obvious that you own all the sites. I've created some big networks before which pass pagerank around nicely. Now Michael Cheney did talk about doing something along these lines in one of his free videos he had out not long ago. I don't have the link but basically what happens is you put the site in the middle of your linking strategy and then ahh i'll just draw it.... Excuse my paint skills, though I'd say they're up their with Michaels! In the picture you put free sites, either blogs or free hosted sites (eg. google pages) inbetween your sites. Then the sites are linking directly to each other, but still are passing pagerank. Ofcourse make those free sites something worst visiting, and highly relevant to your other sites. That's just one option you may wish to look at doing
So you would not have any links going from the main site to the other smaller sites? Only links pointed from the smaller site to each other and the main site.
For me the most important thing is relevance. I dont like dumping links in the footer or side menu. Place your link where it would benifit the users of your website And as terryuk said "Be sure they have different c-class IP's"
I agree with subzero's first point- Relevancy. If the sites are content related, go ahead with the link up.
I think the picture is good. I'd refine it to: Make all sites link to one main site and have a link-page on the main site that links to all the smaller sites.