The only way to truly make sure that your work is not used without your consent is to not post at all...
The next best step would be to damage or stamp the photo somehow... I've seen copyright with name and date digitally embossed on the photo so that the image can be seen and evaluated but would be more less useable. I've also seen little captions/signatures/names added to the bottom of the frame. If you keep it white, it blends in with a forum like this, but if it's dowloaded it stays with the work... Yes you can crop it, but it's one more step... Keep the image size small is another solution. (But having your name ON the work is always the best version...)
Think about this for a second... You go to 1000 web sites and dowload 1 picture from each one... Every picture is one that you liked... Of those 1000 a few digitally put a name or website on the actual work... Which website do you think you'll actually go back to?
I think it's not only a good protection, but a good marketing strategy as well to somehow label/emboss etc. your work...
joe
p.s. With that said, a local wedding photographer once told me that they were shooting a wedding at a house, looked over to see another wedding photo on the wall and it had PROOF stamped across the face of the print. (This was a copy blown up to 20x24 from a 5x5). Some people will do what they want no matter what you do...