I'd announce something along those lines: The next 50 members who register after today and post X number of messages win a prize.
Only registering on your forum is too easy, you want members, but you want activity as well, right? I like the idea of that kind of contest, although I prefer a pure posting contest that doesn't have to do with new/old members.
The last contest I had was a posting contest. The member who will post the most messages between X and Y dates wins an Amazon gift card.