I really believe that there are two very important ways to get the most out of a reward chart. 1. It must be tailored to your child and 2. It must focus on success. I''l try to explain a little.
1. Some children respond well to reward charts because they like to see the accumulation of a large number of things (lots of check marks, stickers, stars, or whatever). They just like to see that they have earned a large number of things. Some children have very short attention spans and respond better to a chart where a larger reward can be earned in a short amount of time. They grow bored with the chart quickly and need a reason to stay interested. You have to figure out what type and length of chart will work for your child.
2. Children only like reward charts when they actually get a reward for success. Asking a young child to repeat a desired behavior 20 times before they receive a reward is too much time and effort before the reward, even if the reward is huge. They need to see immediate results. Try working out a system where they have multiple opportunities every day to be successful and earn a point (or sticker, however you keep track on the chart). Allow them to trade in the points every day for small rewards and also save for larger rewards.
For example, if you are potty training. Sitting on the potty earns a point, pee is two, poop is three. This way there is reward even for trying, not only for doing it exactly right. Have a jar filled with something that appeals to your child (snacks, candy, small trinkets, toys). Each item has a point value and the child has the opportunity to trade their points in every day for an immediate reward or save for a bigger prize (trip out for ice cream, or to a toy store). Allowing the child to focus on all the things they did right during every day not only reinforces the behavior you want to see, it helps the child to feel successful and builds their self confidence.