Just be patient and let the members role in naturally. My mate did no advertising on his forum, None at all after the first five members, And google and those 5 members did the rest, In two years he has half a million registrations.
This is something that should go with out saying, seeing that we are a small forum at the moment. I am currently thinking of ways to go about it, whether it be competitions, or just going on other forums and advertising the forums in the appropriate sections.
Also the hotel I currently use I'll try and get some room advertisements on it with that picture Retroa made and also you should make a badge that if someone clicks on your own add to the forums the hotel gets lift in topsites. :/