How to promote a forum without any money?

[sman21]

How can I promote my new forum without investing a dime in it?

[ksmith29]

Do social bookmarking, SMO, forum marketing in other forums etc.

[batman]

If you are not investing any money in advertising then it may take some time but if you be patient then you will succeed. Start with with posting and forum marketing. As soon as you have built some reputation you will have some followers and some incoming traffic.
Direct the followers to your forum through social media pages and you will see that after some time your forum will start turning. In the mean time build as many as content you in your forum.

[zeus]

I think forum marketing is the best option to promote without money.

[AlyLauren]

Social media is one of the best channels to promote your website, but as what they have said you can off-page optimization. And hey do participate in the thread that will be created in your forum site so that you can build a good relationship with the people who will choose to have a discussion.