The hard part of incentives is to do or offer something that everyone wants and will motivate them to perform. Almost everyone salesperson says money is the strongest motivator, but most salespeople have unlimited earning potential through normal sales and this doesn't always motivate them. I think it is important to get your sales staff's input on what they want to get. If they contribute then it might be something they are willing to push themselves to reach. I had one manager that did this, but he even took it to the next level, by asking each of us what we wanted individually, and the winner got what we wanted (approved before hand of course).
I remember one sales person did want money, but others wanted a giftcard to a restraunt, an office for the week (vs a cubicle), a vehicle for the weekend, . This went on for weeks, once you won you would come up with something else. This seemed to really motivate us.
Another fun incentive was more of a lottery, that was pretty big. Everytime you sold a car it added a ticket to the bucket. So the more you sold the more likely you had to win. There were also multiple prizes so you could win a a couple times.
Other incententives I have seen are team based, where everyone has to meet goal for everyone to win. But this sucks when you do everything you are sopposed to do but another person doesnt pull their weight and you dont get it.
Remember to make goals challenging yet achievable.
Hope this helps.