One method that seems to work pretty well on some Yahoo forums is that new members default to moderated status. As soon as they make their first post (assuming that it is at least somewhat on-topic) the moderator promptly approves it and switches them to non-moderated status like everyone else.

Personally, I find this less obtrusive than waiting for the moderator to approve my membership before I can even try posting. Just be sure to explain to the new member that this is being done to prevent spam rather than as some type of probation or censorship. Spammers are not likely to take the time to comprehend the subject matter of a forum and compose a relevant post.