Today it has become second nature to get an employee in the door, telling him whatever you need to make him want to join you. But when he starts working he sees that it was all a show.
This causes employees to quit fairly quick. A better way of interviewing prospects would be to ask them how they would solve current problems (not giving out too much information). For example: Your in a call center and there are way too many incoming calls for you to handle. What would you do? Or, you have an angry customer that requests to return a product. How would you handle it?
With that said, you have to keep up your end of the bargain too. If you say that an average salesman makes x amount of sales, and it turns out you where misinformed. The new employee will think know you are lying to him.
Make sure to be as accurate as possible, and stay informed about what goes on in your business.