Not all forums are created equal. Some are small highly targeted affairs while others are large forums with very broad topics. Some forums are pro-business and welcome a little self promotion (Warrior Forums is a good example) while other forums are communistic affairs that boot anyone who even breaths about their business online.
You will be making a serious commitment to any forum you plan to use for promotion, so shop like you would for a long term girlfriend/boyfriend. Each forum develops a personality of its own so you and the forum better match up in style, interests, and approach to life. If there is not a good match you will be frustrated and the promotion will fail.
Search “niche +forum” to identify possible candidates to join.
Choose an appropriate non-offensive but memorable user name and fill out your profile with some reasonable detail. Do not include AIM and Yahoo profile links – only spammers do that. Do include a link to your main website if there is a spot for it. Look around at how much detail some of the forum thought leaders put in for a guide.
If the forum allows signatures definitely put an anchored link into your signature. Be sure you understand the forum policies on signatures first though so you do not make enemies with the management. If in doubt, ask first by private message.
Regularly visit the forum, starting good topics and making intelligent comments. Where it fits well into the conversation drop in the occasional anchored link to your blog, but make sure you are not offensive or off topic.
Don’t always link to your stuff, link to other interesting resources too. Score big points by posting appropriate links to the blogs of the forum thought leaders and up and coming members. Others might even reciprocate (but NEVER ask them too).
If you blog, occasionally you can write a killer post with a little controversy. Post a summary and link to the forum as a new thread, asking others what they think. Say something like “If I got this wrong please let me know“. Participate in the resulting discussion, but not too much since your comments can end the discussion if everyone thinks you have your answer now. This technique can be very effective to bring in traffic to your blog posts.
Forget using a forum to promote yourself unless you are prepared to come to the forum regularly and contribute. As a regular poster you will develop trust and make friends who will engage with you in your business, blog or websites. As a rare poster you will be considered a lousy drive-by spammer worthy only of being banned.