I have two different systems that both work.
I make coupons on my computer. When the kids do something that makes me say wow they get a coupon. Ten coupons means a matchbox car or new hair things.If they want to save them up there are bigger rewards.
Little things that they are expected or is a routine then I mark a piece of paper. For every mark they get a penny. I call it the penny point chart. It is an inexpensive way of rewarding good behavior! Good Luck.