If you don't want it to be stolen don't post it on the internet, simple as that (and sad as that).
I don't use watermarks in my portfolio, if you come to my website and look at my pictures and you decide to take them you will take them, no matter what. And you most likely know what you're doing is wrong.
I use watermarks on my event photos where I expect to make my money on prints and digital downloads. I have several versions for landscapes and portraits running through different parts of the picture. I always pay attention not to run it through the potential client's face. It's better to lose a photo than to lose a client for life because you crossed their face.
For photos that I post on message boards I usually put a simple one in the bottom just as a friendly reminder that I took the picture. Again, if you want to steal it you probably will. I make an effort never to post anything larger than 800px on the long dimension. Anything bigger than that will print decent.
