Be sure to offer great content and support to your members. If you are looking for members, then I'd suggest offering free downloads which would entice members to join.
To get more contributors is a little bit more difficult as you want them to be active on your forum. The best thing to do is make sure that discussion is constantly flowing on your forum and ensure your members enjoy using your forum.
Content and activities. Activities can be fun games, contests, projects, and much more! I hold fun activities like Trivia Tuesday and Throwback Thursday often on my site which gives viewers something to look forward to and allows them to engage.
If you are serious about your website/forum/blog and want it to succeed then you must put the effort in regarding advertising. Advertise on EVERY forum you can think of and try to come up with incentives for people to come to your forum such as forum currency or real time cash.
Also make your content king, as I hate when I see forums just blatantly copy each other and every topic is pretty much the same..make your topics unique and it could work out well for you. Offer higher rewards for quality content.