Mmmm yes we were advised to give rewards but never to take them away, once earned that was that and to use other things as punishment. The other thing was with the jellybeans that in the early stages the reward had to be instant gratification, hence 1 bean at a time, maybe you could find something a bit healthier (we don't have a problem with sugar overload DS is cealiac with heaps of food allergies so we struggle to get enough calories in him)! The reasoning we were given is that the time frame for exchanging rewards was too long it needed to be a 'well done' now to get the early training in. (I did think it was a bit Pavlov's dogs, but it worked) and yes he did get wise and after a while if we weren't rewarding often enough (in his mind) he would come to us and say, "you know I think I've been really good this morning, do you think I deserve a few lollies/ice block/coins" whatever was in his head that he fancied at the time and we would have a wee chuckle and agree with him. But hey, he was right and he did deserve it so he was fully trained and now we have moments where I still get a surprise - "Mum, you and dad are really busy can I do anything to help?", "sure son can you go get the bins in?" He disappears and I think oh well nice try mum he gone to his play room, but no 2 minutes later here he comes trundling down the drive with the bins. Yeeha, didn't need to even ask twice.
The thing is to begin with it is a constant repetition of reward and praise and you do have to be doing it all the time but as they improve and get what its all about it becomes less. And if she has been good and is asking for a marble because you forgot then I think it shows she knows what its all about and hey its only a marble, have a big stock and go for it I say.
That's one thing I like about his school when they give out house points they give them in tens, fifty's, hundreds even, what does it matter they're only points and it sure makes the kids feel good rather that have 1 house point - whoop.
I'm not suggesting that this way is the be-all end-all but it sure did work for us and it did come from a kids clinical psychologist.