include a link to your forum in your signature. Use some link building and social bookmarkings and yes dont forget Facebook, make a page and promote it using your own connections.
Depends a bit on the topic... If its local you could promote it with local advertising in a paper or small flyers... Yours is a bit of a strange mix... SEO, movies and general discussion? Who is your target?
I just want to include SEO as a topic, like i also used wordpress discusssion and theme, and blogger discussions and templates, so cant i make a mix one??? please let me know,
You could link the forum from a Blog. Also think about promoting it through Facebook. The more people you get involved from the start the better. Otherwise the forum will seem slow and people may lose interest. You could also link yourself through other forums too if the Administrator is willing to help.
Activity is key so the first few months you need to dedicate A LOT to your forum! Not sure what that is.... Of course you can mix but make sure the topics fit together. And you should have a "red line" that unites everything.
Starting a forum can be very difficult. I would start it off by giving away ebooks and other free information to get people to join your forum. Next, I would have contests for best posts and most posts to encourage discussions in the forum. You'll need to keep conversations fresh in the forums in the first few months to keep people engaged. If you don't do this, the forum will lose momentum after a while, and the visitors will disappear.
Have you looked into Onlywire? Seems to be a pretty good social marketing tool...it is free if you add a link back on your site.
The best method to promote your forum, is to be an active member on other forums with the similar niche as yours. Gain the trust of most of the active members and use your forum's link in your signature. When you befriend the active members of the community, then you can ask them if they'd like to help your forum get a little more active. Also, if the forum that you're using to gain extra members from has an advertising section, then that's better. Just have a nice title of the topic which is advertising your forum and members might click the link.
I think the most important this is to build a community around the forum. Don't over moderate, reward active and helpful posters. You may need to initially seed the forum with basic threads.