Steps to determining appropriate link partners to boost the SEO of your Website.
PR is an important determining factor when linking to another web site. PR0 sites and greyed out PR can most often indicate a site that has been penalized in G. Alexa ranking, however, is not a factor that is looked at at all since Alexa rankings can EASILY be spoofed. However, PR does not change from month to month. It's widely known that Google only updates PR about twice a year or every six months or so. Unless the site is penalized or banned, then the PR is usually zapped to 0 or greyed out. Which is why it is important to review your link partners occasionally.
Relevancy is becoming a more and more important factor now a days in SEO. Back in the day it was purely about mass amounts of links coming from anyone and anywhere. Now, I'm noticing that in more competitive industries, relevancy is highly weighted. However, with some niche sites it becomes very difficult to find relevant linking partners, so in those rare cases links from anyone will do.
Spam detection.
One of the biggest factors we at Affinity Images look at when linking to someone is whether or not they use "spam" techniques. This includes looking for hidden text, keyword stuffed HTML elements such as alt and title tags, sneaky js and meta redirects, PR spoofing, sites consisting for the sole purpose of adsense, scrapers, affiliate sites, and most often sites that are included in the pharmaceutical, pr0n, and casino/gambling industries as these tend to involve the most spammers.
Three way linking.
We've all gotten an email from these people before. You link from your PR5 site to my site A and i'll give you a link from my site B with a PR1. Oh and by the way my links pages are hosted on a subdomain(which leads me to my next point) of site B with a PR of 0. Um.... NO THANKS! Even if it was a good deal I still wouldn't do it, because 90% of the time you'll get screwed over.
Links pages hosted on a seperate site.
If the site that I want to link to hosts their pages on seperate domain/sub domain is usually a no no.
Blocking the links pages or nofollowing links.
Check those meta robots tags, robots.txt files, and watch out for the nofollow tag. All red flags for me. Whats the point of getting a link from you if you block spiders from crawling them??? Unless you're going to provide me with tons of traffic! But honestly how often does that happen? You can download a great toolbar called SEO Quake and check the line through no follow links and it is easy to find sites with the no follow atribute in there code.
***The tip here is that the no follow tag is a great way to boost your on page SEO distribution of link juice.
So who does that leave to link with? Not a whole lot sometimes, but if you've been doin this for a while you know how to take advantage of advanced search queries, directories, and certain software programs.