Right now, I’m thinking of two different strategies to accomplish your goal.  The first one would be to pay attention to your customers’ behaviors, and notice if any of them look like they might be planning to steal from the store.  I can then recommend a series of things to try that might help you prevent the theft.  Another strategy is to have security guards in the store.  I can recommend a few options for what sort of security guards to hire.
assistant
OK, I can give you the broad strokes right now.  Are you familiar with digital cameras, like the ones on your computer?  You can have them point at your store and capture video of anyone who comes in or goes out of the store.  The video can then be analyzed to see if a customer stole something.  You could also have security guards on the premises to watch for shoplifting.  You’d want to install a security system that’s up to date, in order to prevent people from circumventing it, and to get in touch with a company like McAfee, which could help with training your staff to work in security, as well as provide the security system.  Would that help you out?  Does that sound useful?
assistant
How can I prevent shoplifting?
user
Sorry, I’m not sure what you’re asking. Could you tell me a bit more about what you mean?
assistant
If I work security at a store, how can I prevent shoplifters from getting away with their crimes?
user
Security usually involves using tactics like video surveillance to catch thieves in the act, or the use of police to arrest them.  I can help you learn more about how to build and run a security system like this, if you like.
assistant
Yes, give me details.
user