go for social book marking sites to say the world about your forum. Now a days everybody is looking to share their knowledge and link to the world. You can add your site to various sites which displays available forum lists.
Like write an article - Top forum sites of 2011 June or like so. Your site will get notice this way.
Hi, here are some suggestions advertise your forum -
1) You can try submitting your site in all the directories
2) Do submit in bookmarking sites which are effective such as StumbleUpon, Facebook and Twitter
3) You can also tell your friends to join the discussions and contribute to your forums so that others may find it useful.