We have two good answers here.
But there is one obvious one that it is not, may be to be so obvious: ask other sites to link back.
Check over the web what are good related sites to yours, make a list of interesting ones, send an email to administrators/moderators asking to include an article or just a link about/for your site.
If you have quality content, they probably will link back. Keep a tracking of this link building. For who you asked, when you asked, when they replyed, do they have any questions...? Follow up with that.
It is simple, quick, and worth much more than people usually imagine. You need just a browser, a spreadsheet, an email, and a dose of good writing, being polite.