To get promoted you have to be active on the forum between the
15th of the month to the 22th of the monthThen you will get promoted on the 22th of the month
The more active you are between the 15th of the month and the 22th of the month the higher is your chance of getting promoted. (just post every day. respond to some of the threads in the "Off-Topic" forum etc.) - The forum moderators has to notice you.
To be honest I do find this system to be very unfair as you can have hundred of posts on any other dates and they don't count at all.
But this is how EA does it
Where is a forum thread about it in the FAQ section:
http://forum.ea.com/eaforum/posts/list/10224420.page