It is not perfect but here are a few things we do to try and slow down how much theft we have of our rightfully owned images.....
We do three things...
1) make it easy for clients to do the right thing and reorder from us
by placing our contact info on the back of every print sold and attaching copyright info to any digital files sold.
2) We place road blocks or more precisely speed bumps so that it slows down the amount of infringement of our work, like use of Flash Based web pages versus static html where copy with left mouse click is so easy and we always post images to our site with low resolution.
3) We invoice offenders when we find them, then it is a collection issue.
These steps along with supporting the Professional Photographers of America through our membership helps . Education of our client base with a soft and non big stick approach also helps us keep from alienating our customers