Also, make sure the terms and conditions of employment include a clause saying that employees may be searched before they leave the premises (I know, this is almost unworkable in a practical sense, but it is a deterrent).
If you have CCTV, train cameras on the till, staff rooms and stock rooms. There are some sophisticated systems around these days which will record sales and both employee and customer. This will help wipe out fraud, refund abuse, cash theft and 'sweethearting' collusion between staff and customers.
All of the above is of course, textbook advice. But it's never that simple, is it? The couple who contacted me most recently had been told by a regular customer that a member of staff was ringing up a lot of non-sales. When they examined the till rolls they saw, for example, 48 non-sales in a single day. "My wife and I would have maybe five between us in eight hours," he said.
He spoke to ACAS, which suggested a staff meeting, and installed covert cameras. He also thought up a clever idea as the VAT man was due to visit. He told the staff about the cameras (£500 wholesale) and the VAT man's belief that the store owners were "on the fiddle" due to the number of non- sales. Now the non-sales are down to two. They have fired no one, taking the view that the problem has been nipped in the bud. They also avoided that lengthy firing-and-hiring process.
Now all they have to do is worry about the people on the other side of the counter.