When designing incentives to encourage a particular action or behaviour it's worth considering that often the most effective incentives show that the company offering the incentive has gone through some extra effort to provide it.
For instance offering a $25 gift voucher for completing a survey requires little effort or thought, but coming round to clean your windows requires the giver to go out of their way a bit, even though the actual monetary value of the incentive may be less.
There's a clip somewhere online where Rory Sutherland gave the following as an example.
Apparently there's been some research indicating that one of the reasons women like to receive flowers from men is precisely because men feel a little bit uncomfortable buying flowers, therefore the act involved some sacrifice on the part of the giver.
Thanks to Rob, who posted this pic earlier, and which seems to tap into that insight nicely.