Offer prizes or hold a contest for the regular posters initially, it helps.
this isnt good, offering lots of money will only find you members who are not interested in forum or you but just interested in your money.
see a forum like a warez board, they dont pay people to join it, but still people join it.. did u figure out why? the thing is that the forum is indirectly doing benefit to the members and suits their interests but not by offering money.
similarly if you start a games forum, you put some special benefit or make people see the reason why they should join it then they will definitely like to join it even if you dont pay money to them to join it.
see a reverse trend, dnforum asks members to pay up to join it.. why? have u ever figured out? reason is simple.. people see reason enough to do it and so they need to join it and dnforum never asks people to join it and they do not distribute prizes like many forum admins do, so dnforum charging them to join it. because its having its own demand.
while forums which are mere loosers opt the way of luring in people by prizes if they cant offer them some good benefit other than money and these forums cant make people see reason enough to join it, so they opt the cash method to get in short term members, but dude ultimately in the long run the forum admin realizes his mistake and see that he isnt able to build a good community which has a strong demand when he is not paying people see for hostbidder.com it was so good so many years till forum admin was giving free domains, hosting etc and their hb$ was valuable, but since last year it has become almost dead as forum admin doesnt pay anybody and so its predicted to finish off totally by the year end. on the other hand see the dnforum or dp case, had they been offering cash prizes in the beginning then they wouldnt have been able to build a sucessful marketplace where there are rich buyers to buy things of sellers as because the entire forum population would be then consisted of sellers and people looking for money themselves and they wont have much of a status to buy something from other people, so their marketplace cannot be sucessful in the long run just because the forum members are all sellers themselves who are more interested in receiving money rather than giving off their own cash. so a forum without a good marketplace could not be considered as a sucessful domain or webmasters forum atleast as far as i have experience.
so its upto you to decide what kind of a forum admin are you? and ask yourself that if you need people to join your forum or do people will themselves see the reason to join it even if they dont get paid?
choice is yours!