As far as i know, we can earn merit if you post or comment a meaningful and helpful comments and not out of the topics, meaning it is commendable. Second, it is easier to earn if you are not involve yet to a campaign.

Merit in bitcointalk is a reward to your effort from contributing well to a post but it has turn to a thing of privacy which can be given to anyone they know , because I use to see many comment and post that does not even deserve comment but been giving merit .

So I advice to always make a good comment and post it will come your way one day because so many people have good comment and post but have no merit till now with 100+ or even more post and comment. Just do your best and contribute meaningful comment to the community may be one day you can have it, good luck.