The idea is to show them what behavior is good and guide them to choose it.
Each time they do something that you would like to reinforce, tell them "Wow, great job Jimmy! You earned a marble!" Then, let them see you put a marble in their special cup. (Each child should have a cup with their name on it)
Go to the dollar store or target dollar spot and let them pick out 5-10 items. These items go directly into a treasure box/bag that is put away. Inform the children that the items in the bag are off limits.
When the child earns 10 marbles, they can cash in the marbles for a treasure from the bag and this part is important. Make it a big deal that they earned TEN MARBLES! WOW! and let them know how proud you are of them.
In order to get them in the groove, give them a marble for almost anything. You will be amazed at the things they will do to earn a marble. Remember they need to earn 10 marbles to get a dollar treat. Don't be too stingy with the marbles, or they will loose motivation. Later, you can say things like "Thank you for brushing your teeth, but that is something you should be doing everyday to ensure your teeth stay in your mouth". There is a balance to find of course, but I am sure you can do it if we could.
My children are going out of their way to help each other out. I only let them cash in marbles when they both have 10 together. My son has been earning a little more than my daughter so he has offered her some of his so they can reach the reward together. At this, he gets a bonus marble and they both reap the reward of a treasure. When my daughter cries or carries on simply because she has to go to bed, I say "It was unfortunate that you chose to carry on because you could've earned a marble for doing what Mama says the first time. Maybe tomorrow you can try again." This concept works wonderfully for teaching them to do things the first time I ask. Always a learning curve, but they are starting to get it.
I was thinking of other ways to use marbles too.
How about around Christmas time or birthdays when we have to give away toys that aren't played with anymore... we can do 1 marble for each toy you give away! We have yet to see how that turns out.