Website authority is a biggie when you start planning your linking campaign. There are different ways to measure domain authority from the perspective of links, including the quality of the links, the quantity of the links, PageRank, domain rankings and the relevance of the domain keywords.
If a domain links to you that has been in business for a while, and has 100 links itself from other trusted sites, it is more valuable than a site with 10,000 links that are not valuable, or from a site with 10 links. You have to weight the quality and quantity. Paid directories are still a way to go, as everyone buys links from these directories for PageRank and Google still allows it, but this practice is on the way out the door. Paid for links are on the whole not allowed.
PageRank can help you if it is coming in from another site, but remember what you see on the toolbar may not be an accurate representation as there seems to be a several month lag between real time stats and the visible toolbar.
If a site already ranking for some of the keywords you are targeting links to you, that is a big vote insofar as value is concerned. It’s an influential vote – if they are ranking for that term and find you worthwhile, people will pay attention and Google will too.
Some large sites can rank for keywords not even that prominent on their site, but if a site with a domain similar to yours links to you that may help as well. Keywords and anchor text are shown to improve the quality of links, and as noted above if another site in the field fids you worth linking to, others will follow suit.
Bottom line – if the site linkiong to you has street cred, you get some purely by association. Be the kind of site they want to hang with, and choose your heros carefully!