It's only my personal opinion, but you could add a donation button. Everyone could donate and then you could organize contests with super prizes to win.

A good option would be to establish a project (since it's a site dedicated to game development, it could be fx. a 3D engine) where everyone gathered here could contribute. I'm pretty sure that could be attractive.

I'd also move to some other forum engine. phpBB is cool, but there are better around. Even though they're paid, if you follow the first idea, some people may actually donate to help.

Also, if you need anyone to help you make this website better, I'm eager to help.

That's all from me.