Important stuff

1. The theme of the linking website
   This one is very important since the links from a relevant website are worth much more. On your link building efforts, try searching the websites that are close or at least similar to your own site theme. Though having a link from the unrelated site is not bad by itself and even Google admits that a webmaster doesn't have a full control on who and how links to its website. Nevertheless, avoid links from unrelated websites or sites with illegal or unethical content (porn, malware etc).
   
2. The theme of websites you link to
   On the other hand, you have a full control of the links placed on YOUR own website so if you link to some unrelated website - it is you who is responsible for that and it is your site that will be penalized. So be careful what sites you link to. Linking to some unrelated content not necessarily leads to a penalty, but anyways you should be cautious and link only to quality websites.
   
3. Anchor text
   The anchor text of the inbound link is very important and if you can adjust it - try to squeeze all out of it. First of all, avoid using the same anchor text all over the links. Use synonyms, paraphrases, different keywords, whatever else. Second, put the important keywords in the beginning of the anchor text. Finally, do not put all of your keywords into the link. There is really no reason to use anchor text longer than 50-55 characters or 10-12 words. Keep it short.
   
4. Landing pages
   This factor is often ignored even by some professional SEOs and webmasters. It is not enough to simply have a link to your site! The link must be a) relevant; and b) quality. And you must be sure that both websites - the linking and the linked - qualify to these requirements. As for the theme of the linking website - see the pt.1. But the theme of your website should also be quality and relevant both to the donor website and to the anchor text of the link.
   
   Well, it is not necessary in fact, but it helps a lot to have a properly optimized landing pages for every link you have. What this "proper optimization" includes?
   • The landing page must include keywords mentioned in anchor text in its content;
   • The keywords should appear in all important places like the title tag, the headings etc;
   • The overall topic of the page must match to those keywords.
   If a landing page qualifies to all of these - it gets significant boost to its rank, because the corresponding inbound links get much more value now.
   
5. PageRank
   The PageRank doesn't do anything by itself and the green PageRank does even less - it is simply an EGO-meter. However, the PR of the linking website (or a candidate) gives you an approximation of what the link from this website is worth, what value it has. Also, high-PR website are considered as trusted and gets some more value from Google. See below for more trust-factors.
   
   A single link from a PR10 website (if you somehow manage to have one, of course) will quickly boost your own website PR to 7 or even 8 giving you the comparable boost in your search positions. But this factor is the last in the list of important off-page factors, because at first hand you should find a relevant and quality websitethat is willing to put a link to you and then (only then!) check its PageRank. Exactly in that order. Because quality content is worth more than high PageRank.