Five years ago this worked well. I would just write to related sites and tell them about my site without asking for a link and several would link.
Now the same technique either gets an irate reply from someone sick of being asked for links (mind you I don't even ask for one but they assume that I did). More often the email is just ignored. So I decided a while back it's not worth doing.
I don't know what I'd do if I were starting from scratch now. Since my sites are pretty well known they do OK but people just don't seem to link as much anymore.