[DeanLag]

Try to convert those visitors into registered users by building a relationship with them.
Reward users based on their activity. Mods such as vBExperience, vBCredits (Point system), Awards system, do get current users participate more and will also, possibly, help converting those guests into registered users.

If you're going to pay someone to post; might as well try to host reward-based events, depending on what your forums is about.
For example, if your forums is about cars - host an event called 'Post your picture of your car' and the best car, voted by your community, will receive a prize of $x. This will help creating your community and keep them.

Try a referal-based system as most 'friends of friends' will want to stay because they know someone on the site.

Also try limitting guest access. If your whole forum is viewable by guests, they won't have any incentive to register. Maybe show them the first post only or only the thread titles.