I know this sounds simplistic, but the best way to successfully breed box turtles, and herps in general, is to make certain that their physiological and psychological needs are met. If their needs for warmth, nutrition, shelter, clean water, light, humidity, and space are met...breeding just comes naturally. Learn all you possibly can about proper husbandry and breeding will soon follow.
It is suprising that a pet store is having breeding activity. Most pet store box turtles are in terrible condition. Not all, but most that I have seen and heard of.
Keep in mind, some males are more aggressive breeders than others. I once placed a pair of box turtles in a tub awaiting measurements while assisting someone in their research. These were freshly wild caught just a few yards from each other. The male immediatly mounted the female and wouldn't stop even with a camera lens in his face. I have also seen males that will rarely, if ever, attempt to mate.
This time of year, probably the most common cause of halted breeding in captive box turtles is "semi-hibernation". Placing your turtles in a secure outdoor pen in the warm spring months may help.
The good news is, most female box turtles will lay fertile eggs for several springs/summers following just one successful mating.
Unfortunately, it is VERY common for those selling box turtles to incorrectly sex them. Especially with Three-toes, I see them incorrectly sexed on the net frequently. Keep that in mind.
What type of box turtle?