how about 5 points per 10 posts.

then everyone would just post rubbish all day to get a premium account and not have to pay for it , but I think a little something could be done that gives a few coins away as prizes to keep a little excitement on here.

what if there was a game at a random time during the day where it came up "winner receives 10 coins"

isnt alot but keeps the funds coming in, it works on a site called www.tacticsarenaonline.com they get certain drops randomly to help increase there account so could work here too.

Not a good idea. That would end up resulting in a lot of spam posts and the only way to keep it fair would be to monitor everyones previous 10 posts to determine between genuine and spam, which wouldn't be at all viable.

There could be an hourly winner or every 2 or 4 hours selected at random in-game window for 10 points
And/or
There could be daily poster winner selected at random on the forums for 20 points.