SEOMoz has an excellent piece about sending out links requests (something he argues is still of value). He has a 4 step process which he delineates here. Below are two comments of his that I found particularly intersting:

1st comment:

…The best way to determine a link’s value is to see where the page/site ranks at Google in general. If the site’s pages are consistently ranking in the top 3-5 results for the target terms, you’ve struck gold. If it ranks consistently in results 6-20, it’s still carrying a lot of value. If the rankings are highly sporadic and you only find a few relevant pages that rank in the top 20, the value is lower…One big mistake early link builders make is determining a link’s value based on PageRank. I think it’s a grievous error, as you’ll be highly over-valuing older sites and, in many cases, overlooking pages that rank well, which to me is the single best indicator of value…

This seems like a much more solid way of determining a links value than merely looking at the PageRank.

2nd comment

…you have to look at the site/page you’re requesting a link from and ask yourself this question — if I owned that site, what content on my site (the site you’re link building for) would entice me to link to it? …If the page is a directory or takes paid submissions or is linking directly to several of your competitors’ homepages, they’re likely to link to you without any special content considerations. If the site’s a research or informational site, you’ll need to have a resource that provides value to their readers. Oftentimes, a tool, multimedia piece or collection of data/research unavailable elsewhere will be the key. If you’re dealing with a direct (or indirect) competitor or a commercial entity in the space, a compelling piece of content may need to be accompanied by a reciprocal link (if you’re willing to go that route). Although reciprocal link building as a strategy is well past its prime, in a one-to-one link building campaign, it can be extremely valuable and won’t hurt you if you’re doing it infrequently. If the site is a government, non-profit or educational site, content that looks and feels non-commercial can really help, as can linkbait-style content that delivers value to visitors without requiring payment/registration. In some cases, there’s almost no content you can provide that will satisfy the requirements of a picky site owner, at which point, money enters the equation (if it hasn’t already).

I like it, tailoring the content to the link. It has the nice side benefit of diversifying your content and thus diversifying your appeal.