first of all, you wouldn't post an entire 100,000 word book here, would you?... and no one could sell only a chapter or three of a novel... as for short stories, articles, and other short works, the following takes care of that...
second, work posted on writers forums/boards for critique generally isn't in marketable form or of publishable quality... if it was, the writer wouldn't be asking for help with it... so, what good would it do anyone to steal a piece of a work that needs work before it'll be good enough to sell?
third, if you're in the us, all you write is automatically copyrighted when you finish it...
finally, risk of plagiarism/theft is an inherent part of being a writer... if you're going to worry about it, beyond taking standard common sense precautions, you probably shouldn't be one...
check out us copyright protection here:
www.copyright.gov
you can also register and archive your book at:
www.wga.org
and you should always keep your first notes and earliest drafts, as well as a couple of interim ones... that will show you developed the work from an idea and didn't steal it, in case someone claims it as their own...
feel any safer, now?
love and hugs, maia