I manually approve all registrants myself. I run their email addresses through Google to see if the email address is in use on other forums or whatever. I also use
http://www.stopforumspam.com/and run their IP addresses, email addresses, and usernames if I don't see anything on Google. So far, I have been successful using this approach. It takes a little time - I usually get 20-30 new registrations a day.
I would not use the "pay to play" option for registration. It can scare good people away.
