Hold a posting competition and give out prizes for the visitors who posted most.

I think your design or content is attracting users so they come and go off. Incentive is a good idea but also check out as why they are going off. These days people dont have time to wait they should be offered what they are looking if you can give them right on their hand they will just click back or close options of browser.

Good point. Fast download time is essential. If you have a lot of plug-ins, banners and high res pictures you are going to lose traffic.

You must offer an incentive to register. You may buy a great ebook with the rights to distribute, and it has to be really good to build trust.

Yeah, giving an incentive is a good way to gather more registrant. You can also give them a penny if they can refer people.

offer a freebie for registering. I offer an Ebook to my new members plus run contests as well

Ebook is great idea. And if that Ebook is related to interest of the forum people it would be really nice

I agree on this. Ebook also can give you a traffic if it's interested.