There is a truism about incentives that applies equally to Customers and Employees: What goes up can't come down.
When you consider an incentive program for employees, regardless of whether it's a spiff program for Customer Service, a bonus structure for Salespeople, or a shared pool for people within your organization who find ways to save money, it's always best to start conservatively, with the smallest incentive that you believe actually is still an incentive. If it turns out that the incentive is too low (depending on whether or not your business is seasonal, it can take anywhere from 90 days to a year to figure this out accurately), it's easy to lift it a notch or two down the line.
But what happens if you start too high, and have to trim back?
Even if none of your employees hit the incentive minimum, scaling it back will immediately be seen as a negative - as you 'taking away' something (even if no one ever received it). And it will be grumbled about behind your back for months and months to come. Who needs that kind of grief? Especially grief that you caused for yourself?
There's nothing wrong with incentivizing behavior; there's no doubt that, when done correctly, it works. But the 'when done correctly' part is key. Consider carefully what you are incentivizing, and start slow and small.
The same applies to Customers. The author Terry Pratchett tells a story about a bounty that was placed on rats in his fictional city of Ankh-Morpork. To curb the city's rat population, citizens were given a fixed amount for each rat tail that they brought in. The result? The rat population increased, because the citizens were farming the rats to earn extra money.
If your incentive has a loophole or a flaw, I personally guarantee that one of your Customers will find it. In fact, many of them consider it their job to do so. And you know what? They're right, and shame on you if you missed something, because getting it right the first time is your job.
It's important not to create incentives in a void; don't ever let Marketing do it alone. Run it by every Salesperson and Customer Service rep you have; they are terrific at finding holes and faulty logic. Plus, by having them take part, they will also take ownership. How can that be a bad thing? Marketing, Sales, and Customer Service all talking to each other? Fantastic!
And, again, until you know what response rates are going to be, start slow and small. It's no good ordering 500 free iPads to give away for every new Customer signed if 5,000 show up. They will blog, Twitter, and Facebook your reputation back to the Stone Age, turning a campaign that you planned as a big plus into a potentially lethal minus.
Look. Listen. Learn.