Make constructive/helpful posts, and there will be a chance that a user will reward you with a random amount of merit for it. There are no rules set in stone for it, i guess you could say it is a bit random.

Alternatively if you think your posts are very constructive/high quality, you could ask a user like TMAN to merit you, as he is currently doing here. https://bitcointalk.org/index.php?topic=2825523