You should make the forum have ads if the person is not logged in. That should generate some income.
Also a voluntary donation is easy to setup and I really enjoyed the project so far and would be willing to give a donation as I think most would.
Another option would be have paid plugins. For example I have been bashing my head trying to get the analytics plugin to work. I would be willing to pay for it.
Good luck and I hope everything works out for you.