they run a scheme on the smoked-meat.com that works with one time payments.
Think you get larger inboxes, more things in your signature - okay here you go: http://www.smoked-meat.com/contributing.html

Me I work on the assumption that it's members like me who make a forum, so why should i pay :-)

But as long as you don't shove it down people's throats and make them think they have to contribute to join - it can't hurt.