Contests requiring X number of posts and a new topic or 2 within a set period of time worked well for me...
Seeding the forum with new topics from several fake nicks of your own helps get peeps to post.
Topics about current hot news items..especially controversial subjects gets em to post.
and as GFY has proven, ranks according to post count helps to keep members posting...you can also reward top posters with access to hidden forums..pics..giveaways..etc.
I dont know what the general subject matter of your forum is but adding categories for things like jokes..tech stuff..maybe a soap box forum for peeps to rant about whatever is bugging them in life makes for some interesting reading at times..Recipies/Food catagories gets some involved and those posts are easy to seed yourself with a cookbook or 2..
but no doubt its not easy to get people involved posting...easy to get readers but only a small percentage will ever post.
I've run several phpbb2 forums and have had success with all of those..be happy to help ya if need tips or anything.
hotrocket [AT] gmail DOT com