Lots of very good points about how not to spoil the use of devices. I’ve also found that having three or four different incentives that people can pick from when you deliver them also helps improve satisfaction. Nothing like getting to choose to get a little extra juice out of what you’re doing. The only times I find that that’s not true is if there’s some kind of, shall we call it a Talisman, that gets passed around from person to person for great behavior, or if you are trying to establish the incentive as symbolic. In that case, political campaigns have found that getting people to pick between several designs also introduces the element of choice and I think that does buy something extra.