1. Register new accounts
2. Confirm emails
3. Post one comment saying "Hello I'm new here and love the forum. I'll look forward to hearing more from you all."
4. Wait for 1-2 weeks.
5. Now add signature keyword link in email setting under user control panel.
6. Watch your keyword link in the signature get indexed now that you have a post listed.
7. Do not make any more posts period!!!!!!
So being that I've already made a ton of profiles without posts I'm going to go back and remove my signatures and submit posts. Then I'll follow the instructions above to actually get them indexed with my keywords.
If you're strictly looking for a high number of links then you can post a ton of comments on those forums and later once you're done add the signature. Keep in mind that having a lot of posts only shows more links and do not give you more link juice. I have only found that one keyword link shows up per site so choose wisely. Again only add your signature after a few weeks of posting your last comment. Good luck.