Those two links have the best information. The most important thing is patience. Don't go asking for a merit after a day of writing a good review and it not being merited. If it's good enough it will be, unless you ask.
Bah I forgot. You can also get merits in competitions such as the Art Skills Competition and the Poetry Conetest. Those are weekly events found in the Art Music and Writing section down at the bottom of the forums. Win a competition and you will receive a merit.
Write merit worthy comments on games, Examples of these are long lengthy comments full of proper grammar and helpful information on the game, Or how to improve the game itself. Also just like Cholokid said, Check those links...