Some bad linking practices that can harm your site

PR of a site comes only with time. Unless you are too big a publisher that can add tons of content a day, and rather let me say tons of content that your visitors happen to like and link to that, PR comes gradually. In a bid to gain PR, usually webmaster resort to reciprocal linking with just any site. Webmasters, who got PR, resort of selling of links from their sites as higher the PR, higher the income a web site fetches from sale of links. Sale of links, if done under some restraint, is fine, else it may and rather becomes cause of your site losing PR. After Getting PR has to be maintained by keeping up the quality content and restrained sale of links.

A link to your site has traditionally been accepted by Search Engines as a vote for your site. A link from a topic or theme-related site to yours is better than a link from a site having a completely different topic.
Recently, as the trends suggest, search engines, especially Google have starting penalizing a site linking with a bad neighborhood, or say site with absolutely no standing or relevance to theme of your site. It means, if you sell your link to just any site, which is not relevant to theme of your site, then it would harm your site. Of course, this is not to suggest that even a single link sale can hurt your PR, but in way it does harm, albeit to a limited extent. Now, if total number of outward links that site can give are limited then why not choose the sites to which you should link to. Selecting such links may lead you to wait for some time before such sales mature, but the wait may be worth it.
Now as the awareness in increasing about two-way linking, and three-way linking started, search engines have already begun on algorithm to detect such three-way links.
Reciprocal Links - Almost a Waste of Effort
Reciprocal links are still of some value, provided the sites linking to each other are related to same subject in some way. A link from you to someone else essentially deducts one vote from your total vote count, thereby, meaning its value is minimal when compared to a 1-way incoming back-link.
1-way Outward Links, paid or unpaid are going to hurt your site’s ranking
It is like bleeding your Page Rank with nothing in return, unless you are charging for it. This is because Google or other search engines conclude that that they can't trust you to be honest about your site.
Initially there were two web tribes - those who did not know not so much about how things work, and those who knew more than they should. There should also be a flourishing third tribe, who just build great sites with lots of terrific content that automatically ranks highly – To my mind only the third scribe would have the last laugh,