Customers stealing/shopping your proofs around is usually NOT a problem if you have stuck to your guns and gotten a down payment BEFORE you send them the proof.
If it's a new customer and they have reservations about paying before seeing a proof then you let them come to your place and view it on your
computer or make a down payment before letting them leave your shop with a copy of it, either printed or digital.
If you follow those rules, you will generally never have the problem of someone stealing your hard work and giving it to the low bidder, at least not on your new or prospective clients.
Just don't let them pull the old "I wanna see what you can do before I put any money down" trick on you by sending them an email with your FREE design work attached!
Personally, I really hate sending someone crappy, low-res designs while expecting them to be excited about it enough to give me money. Most people will have a hard time looking past the crappy , low-res image you just sent them and if you try to explain to them that it's because you're afraid that they'll take it to another shop for a better price....well, that doesn't exactly give off a sense of confidence on your part now does it?