1> Web site that is well laid out and easy for any one to navigate.
2> Good content updated regularly.
3> Good moderation keeping the garbage posts and threads under control.
Content is king in forums. If your content is garbage you'll get nowhere quick.
Link swaps work only if the other guy posts your link. Beware of the ones that offer if you put theirs up first. Give 24 hours then yank theirs down if you don't see yours at their web site.
Link in your sig where permitted. Ask a mod or admin if it's cool to have a link to your web site in you sig. Be ready to take no for an answer. Be a little active at the forum where you have a link in your sig. Webmasters like me don't cotton to one or two posts or threads then poof you're gone.
Sign up for Google's webmaster tools. Big help.
Sign up at other search engines and sit back and enjoy the crawler bots scoping you out.
And no forum gets big or sucessful over night. It takes time and patients.