I strongly suggest you don't.
Forums are, by nature, a place for discussion, and articles do not in general lead to good discussions. Posting anything with the intention of promoting your site (by way of links or whatever) is more likely to get your account banned than anything else. Also, posting the same article on multiple forums is unlikely to benefit you, but will very likely annoy the forums.
If you have written a good article, then post it on your own site, where you will get the full benefit of any visitors interested in reading it, and any boost to SEO. Why would you want to give another site the benefit of your work?
Forum posting may help to drive traffic to your site, if you are a regular and helpful contributor, but it is unlikely to have a direct effect on SEO. Remember, reputable sites will mark user-submitted links nofollow, as we do here. Remember to check the forums' rules before posting to avoid any problems.