I read somewhere that Steve Jobs was an underachiever until his dad used money as an "incentive" for good grades. His grades improved drastically and then he was skipped.
All kids have different motivations and when a kid is smart enough to talk their way out of ever doing anything the least bit difficult I think it's appropriate to find alternative "inspiration". My kids never have nor will accept "because I told you." So, I devised an incentive program.
Basically, I bought a bunch of junk they wanted really bad. Mostly stuff I would never agree to buy them simply because it is junk! I stuffed it in a bag and made a chart for each of them. I told them when the chart is full of stickers, they can pick out whatever they want. This way, I never "make" them do anything. They are in total control of completing the tasks, totally on their own time table. They love this. The beauty of it is that I decide what warrants a sticker. I've taken things that aren't there fav's but I think are challenging and made them the tasks they need to complete in order to get the stars. So they think they are in control, but really I am! These tasks aren't all academic either. Straightening up their room to my specifications is EXTREMELY challenging for them! Everyone is happy. I get to see them stretch themselves and work outside their comfort zone. There is no arguing or pushing involved because they decide when they do the tasks and choose the tasks with the caveat each task can only be completed once per day.
And they get a "feel good" moment for doing something they might avoid simply because it is a little difficult. I know, it's somewhat Pavlov's dog, but aren't we all that way. My husband says he does a great job at work because he takes pride in his efforts and that should be the internal motiviation we should be looking for in them. I asked him if he would still work his butt off if they stopped paying him. And come on, they're 5 and 8. Webkinz are THEIR currency!
