Don't link a lot to external sites

Linking to sites not on your site is a good idea, but don't fill up your pages with them. At best, you will dilute the effectiveness of your page in the search engines, and at worst your page will look like a list of links and get slightly penalized by search engines. Also, when you have lots of external links, you have more to check on a regular basis, to make sure that those pages don't go bad or turn into "bad neighborhoods".