I would not suggest trying this unless there is a really good core community there. I also think that doing this won't work with all communities. A good first step is to start with a paid sponsorship which, say, eliminates all advertising on the site. See how much interest there is - ie the % of users who feel that contributing to your site financially is worth it. Go from there.

I would echo that, one sure fire way to kill a community over night is to go paid if people don't think it is worth while. Ask yourself how many paid sites are you a member of?

I think paid will more and more become the norm, it takes a lot of effort and time to run a decent website and the paid ones I am a member of tend to be better quality. Test the market and offer a no ad site for paid members. Then see how things go.

I don't think paid membership would work unless the forum does provide something unique and it's very developed. Stick with free and have ads

Any site is only as good as its content, administrators and moderators....I honestly believe that paid sites will never gain ground on a well contented free site.
We are bombarded with so much 'pay to play' content on the net these days that most people will just look elsewhere for their fix.
The true measure of any site still remains the number of hits it receives which remains the core interest of where companies will place their advertising dollar.
Some of the best sites available to me, personally, are FREE and will always remain so!!!

I suppose you could charge for a unique service which no other place provides. But apart from that I wouldn't really reccomend it, what about asking people to donate money? you could use paypal as a donation system.