What brings merit is quality post but the quality of your post is subjective which means it depends on the merit source spread merit based on what he /she considers as quality post.
A quality post can either be lengthy or short but it's more about the information that it contains. Sometimes your post can be very informative but it has been treated or it's known to the people already hence they do not consider it anymore.
But perhaps you want to earn some merit for good post then you have to look at how people who have been earning some good merit does it and then if you follow the steps it certainly bring you some merit too.
In my early days here, I discovered something about merits. I discovered one earlier and the other later.
What I discovered earlier was;
When you have the consciousness of getting merits and put good efforts in your posts, you will earn it. This worked for me many times, but at a time it stopped working for me. No matter how merit conscious I'll be, I will not get merits.
The second phase came;
This was when I started flowing with the forum discussion and never had merits in mind because I already earned enough to take me to Snr member. Merits started coming gradually and was steady. So, merit is just dynamic.
I understand such situations, and I think I'm currently in the second phase where I don't longer search for merits, although I'm not up to a senior member rank. I feel more relaxed now and a bit more experienced, so I'm not as merit conscious as before, when I used to crack my head on what to do to earn merit because almost everything I thought of was already done before hand.
Now, it seems I understand the merit system better, so I don't force myself on merit sources to earn merit, but I just do my best by contributing continuously regardless of whether merit is coming or not. Also in my local board, I never thought I would be receiving merit from there because I wasn't acquainted with how it works, but at one point, I woke up to two merits from @hugeblack in the local board and @CryptopreneurBrainboss for honestly speaking my mind. From that day until date, I have not been chasing merit because I no longer believe that when it's time for you to earn merit, you will, no matter what.
So what I just encourage other mates to do is keep the stone rolling. You may not be noticed today, but at least tomorrow is another day.