The Ph.D Link Building Course - Ask For Links
There is always the option of being courteous and discreet in your link building efforts and simply making a polite demand to be included on pretty well anyone's website. If anyone you know personally has a website with a reasonable PageRanking, then it goes without saying that they should be pitched to have a link to your website placed on theirs. You may find that they will agree even if the subject matter of your site doesn't perfectly sync with theirs.
Another great source of possible links is from the various contributors to your own site. It is quite likely that they have some form of reasonable PageRanking on their own website and would certainly feel obliged to provide some degree of linkage to your site.
No matter what the industry or most detailed sub-grouping, you can bet that there are a large number of commentators and bloggers who discuss all of the events that occur within that community on a constant basis. Sometimes one is left to wonder why anyone would want to know all the details of an industry in that minute detail unless their very own livelihood is counting on it, but that would serve to disregard the rampant phenomenon of fanboism, where all sorts of otherwise rational people spend many hours a day on technical sites arguing and flaming over the latest 32 nanometer shrink efficiency and implementation of High-K metal gates on their favorite processor.
These commentators and bloggers are an excellent source of linkage, as some of them have fairly high PageRanks and they may be followed intently by some of the more rabid fanbois.
The best way to pitch any of these types of individuals for their links is with proper diplomacy. A friendly yet businesslike email congratulating them for managing such a quality site, showing the relationship between your site and theirs, and then asking if they would consider a link, is usually the best procedure. You may find that if you approach a fairly large number of people in this manner you may get a surprising percentage of results, however that will be primarily based on the specific types of websites they operate and how persuasively you have stated your case for linkage on their site.
It is fairly inescapable that if you are asking someone for a favour, you are imposing on them. Since you have made up your mind that you are going to ask them for the favor of linkage on their website, you might as well just gird up your courage and do it without wasting time or beating around the bush. It is highly unlikely that the other party is going to volunteer to give you links on their site if there is a limited benefit to them. They likely won't even think about it.
You should also acknowledge that it is a favour that they are doing you. That is why you should never demand anything or exercise anything but the most courtesy you can muster. You must also put the other party into a position where they know that they can say no without causing any damage to the rest of your business and/or personal relationship. They have to have a way out and the last thing you want to do is to put their backs up against the wall.