How is discipline, behaviour and general 'effort' in your house? Mine is slightly different now there's only one teenager here with me - all of a sudden it's LOTS of fun, it's very girly (as it's just me and Elise) and there's no sibling rivalry or tormenting to deal with...hang in there, if you've got a way to go yet, I can thoroughly recommend this bit!
However, my youngest is going through an interesting phase and his poor father is bearing the brunt of it - a six-year-old Jekyll and Hyde character is NO fun to deal with, even if the Hyde persona only appears fleetingly.
I've always been in favour of 'reward for good' rather than 'punishment for bad' - which is okay if your child responds to reward, my eldest son wasn't fussed either way (and still isn't!) which made him very tricky to deal with - so this Reward Jar idea is something I would have tried back in the day; I'm going to suggest it to Edward's dad too.
Basically you have a chart with a sliding scale and a marker to show where your child is on that scale for the day - bad behaviour or attitude will see them slip down the scale, general awesomeness will see them move UP the scale.
When someone reaches the top of the scale and stays there until bedtime then they get five pom-poms (or whatever you want to use) in their Reward Jar; when the jar is full they get a prize or something that they've been hankering after for a while.
Whilst you're all decorating your jars and scale markers, you could also draw up lists of potential prizes for filled jars.
Good luck if you're in need of something like this and do let us know how it works out for you!