I guess with digital a clever person could copy anything. I would first have my printed releases, that I give the customer and have him sign, stating the proofs are not for any reproduction. The signed release agreement should be enough to scare them away from stealing and if not, good for any lawsuit. I have found that sending a release form to sign and have them return it, has solved any potential problems for me so far. I have pdf versions of all my releases that I can type in the clients name and details. I can either send an email one and have them print and snailmail it back or have a printed out copy for them to sign.

I would use a pdf (Adobe Acrobat PDF maker software) with copy and print protection on it for sending proofs to the client to view. If they are clever they could make a screen capture of the photo but you could also have your copyright notice splashed across the photo, also make the photo fairly small.