I will tell you how I advertise my forums, I hope it helps.
Step1: Google search, Forum Promotion.
Step2: Click on, Forum Promtion.
Step3: Register in Forum Promotion.
Step4: Look for, Basic Promotion.
Step5: Adveryise your Forum There.
Step6: Make sure your Forum is atractive, (Have fun stuff to do on your forum).
Step7: Make sure you have posted on every thing in your Free Forum.
Step8: Hope you enjoy
http://phorum.free-forum.net