Its not just about getting them to visit, you want them to stay!
The secret to an active forum is simple.....An interesting subject matter
a site can have all the information in the world but unless atmosphere, debate and chat is interesting they will simply skim their required info and off they go!
a good forum subject is one that people ask questions of, a very deep and varied subject will create a very deep and varied forum experience, a subject that is always changing and evolving will really help too
also from my experience the atmosphere is very important, if you have too many rules or are trying to force a code of conduct on people it creates a negativity that ends in negative posting and attitudes towards the staff who are forced to uphold the rules
I run a carp fishing forum which is a massive subject and one which is constantly evolving with new tactics and techniques as well as the massive industry and media side of things which really promotes thought and discussion. people always like to brag and show off and in fishing forums this is a massive plus
oops im ranting on here LOL
also one little point....Facebook is your friend for advertising