The obvious answer, and one I use, is don't post on the web. Unfortunate, but that's the way it is. I share prints - all RC Ilford - with friends and post them in the community, including the post office and the fire station [with permission, of course]. I will give prints away to people who ask, but I see no reason to encourage thieves.
Don't post on the web then there's no exposure for potential sales.
I did a series of documentary image over a three year period of the moonshiner Popcorn Sutton. Popcorn took his life rather than go to prison for the third time. He gained a great deal of attention and the day before killing himself sold his name and and process to a distillery.
Over the three year period I made hundreds of B&W images that have become very high demand images. I sell use regularly to numerous publications like Maxxim, New York Times and Gourmet Magazine to mention a few. Im negotiating with a restaurant chain at the moment to sell images to decorate the bar in their restaurants. I've sold them to other restaurant / bars as well.
Let me say my images are registered with the only right office so I can legally take action if needed.
No web exposure, no exposure to sales. On the web images are vulnerable to theft. If you're on the web and you have images people want they will most certainly take them.
I regularly do searches on eBay, YouTube and other sites an find my images all the time. I've found three companies, one US, one in Italy and one in the UK making T shirts with images taken from the web or simply scanned from magazines I've licensed to use them. I've found several professional music videos including a Grand Old Opry Star using them. This one I thought I was going to have to take to court but the musicians manager got the images pulled.
I've found people making greeting cards , buttons and other items. I also found two billboards with the images. In both cases I collected compensation and in every other case was able to stop the reproduction of my images.
Unfortunately if you want exposure and want to sell your images you have to take the risk and spend substantial time catching people who use them illegally. You also need to register your work and get an attorney and use the legal system if needed.
I recently busted two individuals reproducing prints and selling them through flea markets. Unfortunately people and companies don't understand copyright. They think if it's on the web it's free to use.