It can really take a long time. There are 1000's of poker forums out there and it often takes something special for people to switch. I would look at the forum as something your site visitors can use if they want. If your site is interesting enough to keep people around for a long time, the will just naturally go to your forum. Or if your forum has things no other sites have, they might stick around.
You can always try a forum affiliate program. Depending on the forum software you are using, you can probably find an add-on that will allow you to pay people for posts and recruiting other members. or have a point system and give away real prizes like Cash, Ipods or something like that after people reach a certain point level. Obviously you wouldn't want to give an Ipod away to a spammer so you need to be careful of this type of promotion.
All and all I would say forums are the hardest thing to get going, unless you have a special niche in the industry... If you are catering to a specific crowd then you will just get members, but if you are advertising just poker to poker players, they are most likely all ready posting at other forums and don't want to waste time at a forum with 20 members.
Good Luck
