There are 101 ways to increase registrations for your forum but main way is using SEO (search engine optimization)
My forum has new registrations per day and it is maintained by organic traffic which will make your forum more activities with natural posts and members.
If you want to learn SEO, I would recommend you to learn from some guys here.
http://www.webmastersun.com/forums/1...e-Optimization
If you don't want to use SEO because it consumes you more time, then advertising is a good channel to go with but it will cost you more money. Choosing which way is depends on you.
For me, SEO and advertising on high traffic sites will help to increase traffic and registrations for my forum. It also will work for your forum.
Hope it helps!