This questions, or a variant of it, gets asked once a week here. Search the forum and its archives to get more detail. General pointers would be to: i) compete on a few good traffic keyphrases for your niche; ii) sign up to similar forums in your niche and make valuable contributions, occasionally and tactfully referring to your forum - befriend members of the forums and invite them to yours but be warned this is a slow process; iii) ensure your forum offers something a little different to the competitors in your niche; iv) make a significant daily input to your own forum, posting original material, welcoming new members, and trying to get them to post by asking open but not too direct questions - if you show a little friendliness toward all new sign ups and you have a little charisma it goes a long way; v) don't let any posts go unanswered, but refrain from posting worthless responses to genuine member contributions; Other techniques to get people to your forums might be specific to your niche.
Deploy search engine optimization for your forum. But be prepared. If the dining table is not yet ready and you start calling on your neighbors to dine with you, disappointment will ensue.
This is an excellent point, eloquently stated. Getting people *to* your forum is a very much easier than from getting them to stay and participate. If you arrange your thinking to account for the distinct nature of these two tasks, you will be part the way there. Essentially, you have to engage users, give them a reason to return.