From my experience, sales people need to have a large financial incentive to get motivated, therefore small retainer, open ended commission.

Also, I've never liked flashy sales people. They may say all the right things, but to me this isn't sales. Sales is relationships. Anyone can sell something once and be unlikely to sell to the client again. A good salesperson will not only sell the product, but also build the relationship. This is vital.

Finally, most sales people believe they can sell anything, irrespective of what it is and therefore put no effort into learning their product. As mahmud mentioned, product knowledge is key. If they aren't willing to learn your product inside out, they aren't in it for the long run. Ask them about the last product they sold and ask for details on it. If they can only name base specs etc, they don't have the attention to detail required.