Put your personal information and copyright statement in the EXIF data hidden in the JPEG file header. Flickr preserves EXIF data if somebody copies the file or downloads it. Your name will be downloaded and stored inside the picture. Most people won't even know that it's there.

If you "need" to use a visible watermark, either fade it back or put it closer to the edge of the image.

I often use various techniques to mark my images but, when it comes down to it, if somebody's going to steal your images, they'll do it regardless of whether the image is tagged or watermarked. A good Photoshopper can rub that watermark out in less than five minutes.

Besides, you have the negative... Proof positive that the image is yours.