Be aware of a page’s status

Nofollow links aren’t the only ones that will be invisible to the search engines. If the page is no longer on the index, the links it contains will be of no value for SEO.

Google’s index is constantly changing, and you have limited control over the indexability of other people’s pages. When looking for links, check to see how the other site usually features its links. If it has elements that will block search engines, like the use of Flash, or puts links on a page that’s deep within the site, a link is unlikely to get you much attention from Google.

The best way to defend yourself in this regard is to get links from sites that are aware of their own SEO, have a decent ranking, and generally aren’t likely to drop off the index any time soon. Protect your links by staying away from sites that break SEO rules.