From what I have seen, simply posting more does not increase your chances of receiving merits. Quality is still the primary requirement.
1. I think the first post in a thread is more likely to receive merits, so starting more threads may increase your chances. However, quality still seems to be a necessary requirement, so a single post with thoughtful, insightful, and valuable content is more likely to receive merits than any number of mediocre posts.
2. I believe that a reply posted earlier in a thread is more likely to receive merits. Replying early gives you the chance to state something that hasn't already been stated. Also, once a thread winds up or starts filling up with worthless replies, readers stop caring about it. Take this reply, for example. It might a quality post, considering the time and effort I have put into it, but it is #33, so I doubt that many people will read it, and even fewer will consider giving it merit.
3. Don't bother posting in "mega-threads". There are many mega-threads with hundreds of replies and none of the replies receive any merit. Anything you might want to say in a mega-thread has already been stated many times, and nobody cares about an opinion that has already been stated 100 times.
4. You will easily exceed the activity requirement for a rank long before you reach the merit requirement. Unfortunately, many people have not yet figured that out and the forum continues to be flooded with worthless posts.
Anyway, that's how I see it.