I've got a big bucket of marbles and a small galss canning jar. The boys (Andy & Ryan - Colin tried to eat the marbles) get to take marbles out of the bucket and put them in the jar when praise is in order. The amout of marbles is up to us and depends on what they did and how hard they try.
We used to use a system of pennies. Each kid had a jar and we had a jar. They lost pennies for mis-behaving, but could earn them back. It became focused on the negative behavior, not the positive and really just backfired, so we abandoned it.
The marbles are strictly POSITIVE reinforcement. It forces us to remember to give them plenty of praise and it is a form of praise that they can see and touch. They share a jar, so, they BOTH have to behave to get the reward. We've had issues of one instigating just to get the other in trouble (they both do it).
Once the jar is full, they will earn their reward - a trip to the book store and $15 to spend - each. Now, that could have just as easily been a trip to the Public Library - and it almost was. BUT Andy is reading the Andrew Lost series and is on his last book. We would be buying him more anyway... Ryan and I are reading The Magic Tree House on loan from his cousin, but I want him to get something he can read on his own. He has been reading to Colin, which is great, but he is reading "baby" books that are well below his abilities.
While we are on the topic of kid books, Barnes & Noble is having a summer reading program for school-aged kids. they read 8 books, fill out a "reading Journal" and bring it in to the store and get a coupon for a FREE BOOK! SInce my kids are required to keep summer reading journals anyway, this one is a no-brainer!