If you set up a google alert with your exact keywords, then google will notify you every time anything new appears on the net with these keywords, you go to the site and leave a sensible comment, you then get a relevant, targeted backlink. Most of the blogs etc. will be moderated, so it's no good leaving a stupid three word spam comment.
It works like this:-
If your keyword is 'how to train a monkey to dance' you set up your google alert (follow the instructions on the google alert page).
Each time a new article/comment anything containing this keyword appears anywhere you will receive a notification from google.
If it is relevant and you can post, then post, ping and wait for the search engines to find your link.
With 'normal' keywords you can expect to get about 50 alerts a day, maybe half of which you can post on, so 25 new targeted, relevant backlinks a day for a few minutes work.
Try it - it works.