Some people are thieves by nature.
Some are thieves by convenience
And some are thieves by spite.
Those who will rip off any job they work at, one solution is a security camera, or STRICT keeping of money and inventory, but even that will let certain things slip (and its a lot of work) . Plus, employees will try to find the one place that doesnt have a camera. The best real solution is to thoroughly check someone's references. Even if you're a gaming store hiring high school or college students, get references from someone they know.
Thieves by convenience are your borderline types. They might steal pens and stationary, or batteries and the like (in the case of a gaming store...DICE), or other small objects that everyone uses and are inexpensive that they won't consider them theft. The best way around this? Decide what you consider real theft and tell the employees. Is it ok for them to bring home a pad and pens once in a while? And of course, make sure there's a GOOD employee discount. If you can get the inventory at COST, there's no reason to run the risk of stealing. Yes, I said cost. Don't treat the employees like customers, treat them like YOU. Merchandise at cost won't lose you money, and won't gain you money, but will keep your employees happy. And happy employees are good employees.
Thieves by spite are the nice guys who want to rip YOU off specifcally, because they feel that you're ripping them off. The solution to this is fair compensation. Don't pay minimum wage to full-time employees. Make sure that any positon that has more responsibility has more pay. Make sure full-time employees get health coverage AND vacation days. In this day and age, there is no excuse for an employer to not offer these things. Have some way for employees to air their greivances. Bitter and angry employees become spiteful thieves.
Even with these precautions, you're likely to find the occasional person who thinks you're a softy and wants to take advantage. That's when you bust them. HARD. If you do this much, you know that any thief deserves his punishment, and don't have to worry about it.
Some of these solutions are expensive. Health coverage, vacation days, security camera and good pay. The alternative is to not offer some or all of them, and instead accept "some" level of theft. You have to decide which is more important, higher profits or a well-run store with dependable employees. Maybe even start the store with a "middle-ground" with the promise to take care of the employees if the store suceeds.