We all know that getting lots of inbound links is a critical part of search engine optimization. And most of us know that joining link farms, which create lots of links between totally unrelated sites, is not a good way to do it, because if Google catches you at it, they'll penalize you. So how do you go about getting links without risking getting penalized? Here are a few tips.
1) Only trade links with sites that are at least someone related to the topic of your site. This isn't a must--you can build PageRank by getting links from any sort of site, and that by itself is not dangerous--but inbound links from related sites are much more valuable because they also contribute to Google's assessment of your site's relevance to your target topic. Also, if your link pages point willy nilly to every topic under the sun, your visitors are going to see it for what it is rather than valuing it as a resource for finding topical information.
2) Don't let the links from your site out of your control. This is extremely important. If you use a tool that enables someone else to inject links into your page, you could unwittingly end up linking to sites that have been penalized by Google. Linking to penalized sites can result in your site being penalized too.
3) Keep an eye on the PageRank of the sites you're linking to, and immediately delete any links to sites with PR0. Take a look at this page that I use to track the PageRank of some of my webpages and sites that are sponsoring them to see one easy method of tracking the PageRank of multiple webpages. Better methods probably exist, but this certainly works.
4) Don't get a gazillion new links all at once. You don't need to go too slowly, but a sudden extreme burst in the number of inbound links to your site may be interpreted by Google as the result of participation in a link farm.
I'll finish up by pointing you to a site that I recently signed up on, Link Metro. Some people there are clearly willing to trade links with just about anybody, but you have complete control of which trades you accept, and you can even automatically block link exchange requests by website category.