You can lose Page Rank and Search Engine Results by Linking to sites that are considered Banned, or Bad Neighborhoods. Be aware and check before linking.

Be aware to whom your site is linking to via outbound links. Periodically check your outbound links making sure that they are:
1)Still an active website
2) Still a resource for the reason you linked to them in the first place
3) They have not changed theme formats and are still a quality site.

Page Rank grows over time. Google updates visible Page Rank infrequently, like every 4-8 months or so. The best SEO strategy is to link to and link exchange with like themed sites or quality site.