is already automatically under copyright as far as I understand, whether or not you place a copyright symbol with it. But maybe just to remind folks that it is under your copyright, place it there in posts that you feel others taking bits of or the whole thing would upset you. There is no way to prevent others taking your work and doing what they want with it really but to hope they have some morals. Reminding them to get some morals may be good *L*

You could also contact the places you see your work used without permission at and insist they remove it from their site and/or credit you properly.