iv been a member of a couple of fourms. they all go through stages, high where u get lotsa posts, which generally occurs at the start. then low, where it dies down, this shows who the core group of posters is, then the gradual rise,where it gets steady, but never up to original high.
this forum is on a topic which is forever relevant, so there will always be content to discuss, dw abt dwindling posts, your core member will carry you through. and yes, while offering incentives for getting people to join is a good idea, a better one might be to offer points to people who have a person join up and post consistently for a set period.
on another forum im on we have a guest and a members area, if you post consistently (2+ posts per day) for 3 weeks you are accepted as a member as we can see you will participate in the community.