My best bet would be to save up as much money as you can, because advertisement will be very expensive if you are trying to get high traffic fast.
If you are working on a long-term forums, then just spread it to friends and tell them to spread to other friends, and then your forums will gradually grow. Also, do competitions to keep member activity high such as, new competition as 10,000 total posts or 200 members, or on a set date.
Advertisement is always different for different websites. Some require more effort than others, and seeing rs has been dieing slowly overtime, it may take more effort to get high activity than other gaming sites.