Should you be worried about posting your work on writingforums.com?
I can't assure you that people won't try to copy your work, but if they do you can sue them. This forum's database can be used as evidence, and the automatic copyright law* would be in effect. If you posted the story before they got it published/posted/copyrighted, then you can sue them, as your automatic copyright would be in effect before their's.
Any writer who steals another writer's work is risking their reputation as well as being sued. Also, people usually don't post their best work, because if they don't need help with it, why post it here asking for advice? And people usually only post a small portion of their work, so there really isn't much to steal, anyway.
So there really isn't an advantage to stealing someone else's work off these forums. I wouldn't worry about it.
*"Anytime that you create a creative, copyrightable work and it is fixed on a tangible medium (ie, paper, videotape, etc...) it is copyrighted. You don't have to register the work to have copyright protection. Altough registration brings other benefits such as the immediate right to sue and potential statutory damages, notwithstanding actual damages. So in sum, once you create it, it is copyright protected."
“…you can make the copyright symbol by holding down alt and then typing 0169 on the number pad
-For $30 USD, you can copyright your work with the Library of Congress.
-The real difficulty, though, comes not with copyrighting the actual work, but the ideas.
Copyright term and Public Domain. (Note: Just because something is considered public domain shouldn't give one the right to pilage it at will without giving some due credit to the original source. Call it 'Writer's Etiquette'. See also, Homage vs. Rip-off.)
NOT OKAY to say, "He grabbed a Kleenex and blew his nose."
ALSO NOT OKAY to say, "He grabbed a kleenex and blew his nose."
IS OKAY to say, "He grabbed a Kleenex facial tissue and blew his nose."
As long as you don't diminish the brand, you're okay legally. Always refer to what a Kleenex is, and always capitalize the brand.
Of course, that just looks like product placement to me. I prefer:
"He grabbed a tissue and blew his nose."
Also see, http://www.copyright.gov/ (courtesy of Oz)
http://www.sfwa.org/writing/copyrite.htm (courtesy of Creative Insanity)
UK Copyright law
Austrialian Copyright law