When considering optimizing your site, obtaining quality, inbound links from established, high quality sites is a must for long term search engine rankings to take hold.
Some factors to look for (in no particular order of importance):
The age of a domain plays a big role in the value of a site. In many cases, links from these “aged” domains carry a lot of weight in search engine algorithms and are considered valuable links to have pointing to your site. See a list of the 100 Oldest Currently-Registered Dotcom Domains. The oldest is way back on Mar 1985, symbolics.com. Amazing, considering that no one knew the internet would have such a profound effect on our lives…
Generally speaking, such sites also have a strong established Google PageRank and are authority sites. Links are considered as votes and an old established site linking to you boosts your credibility and trustworthiness.
Zoltan Gyongyi, Hector Garcia-Molina, & Jan Pederson wrote about the concept called TrustRank in their white paper on TrustRank [PDF]. High TrustRank sites making up the seed set unsurprisingly hail from government (.gov, .mil), educational institutions (.edu), and major corporations. Authority sites are currently shielded from search engine problems such as link non-relevancy.
TrustRank helps to
This concept of TrustRank would certainly aid in identifying which sites are worthy of ranking in the search engines and minimize the spam we see in the SERPs.
Obtaining links from .gov’s are near impossible unless your site offers a government endorsed product or service. The same goes for military sites, .mil’s. So far, such links have not seen to be available for sale. Educational instutions, on the other hand, have offered advertising space on their sites for advertisers interested in purchasing links on their pages. Advertising on university .edu sites have drawn heavy criticism for having unrelated and spammy links like timeshares, casinos and car rentals.
Other excellent ways to obtain quality links from established, aged sites would be through press releases and article submissions. Also, instead of trading text links, you can also trade content and keyword-rich articles that will drive targeted traffic and increase your link popularity at the same time. Articles also have a “viral” effect in that many webmasters will syndicate your articles on their sites creating additional inbound links from content pages.