Here's our system. Each kid has a base allowance. They are expected to do their chores and help out whenever asked for this base amount.
We then track +'s and -'s each month. They are free to do anything above and beyond their 'usual' expectations for more money. If they do extra chores or helping without us asking that is a +. If we have to nag them to do their normal chores that's a -. If they are miserable humans to us or each other -, etc.
At the end of the month we then go through the list - lots of +'s and they get a bonus on top of their allowance, lots of -'s they get less. Most of our stuff is around pitching in around the house. We haven't really had the need to extend beyond that (ie school or activities) but it would be pretty easy to adapt to pretty much anything I'd think.
