I have to kind of agree with Davy's point about the kids wanting to help. I mean, you can only "make" children do things for so long. Eventually they get old enough to put their foot down, and usually do so with a vengence. No one likes to be told what to do like they're servants in their own homes.
With my step-kids, we made a list of chores, responsibilities, and house rules. They get marbles for doing the chores (more or less depending on the job), they lose marbles for breaking the rules. The marbles can be cashed in for rewards. They choose how much to do, and how much they want to make. When my husband and I need help with something we ask for a volunteer. Sometimes the volunteer gets marbles for helping, and sometimes they don't (the whole pavlov thing). They climb over each other to volunteer because they know they might get rewarded.
If they want something big and a birthday/christmas isn't close they can choose to earn the money on their own. We help them make a plan of how much they want to earn, and then what chores they will do to earn the money. They do the chores because they want to, not because we make them. But, if they choose not to participate in the household duties they don't get "extra's" - like playing video games, money for allowance, candy, etc.
As they get older some things that they get paid for become things that they get decutions for if they don't do them. For example, brushing their teeth. When they were little they got a marble if they brushed their teeth without being reminded. Now they lose a marble if they don't do it.
We believe that this mirrors life when they leave the home. We all get to choose how hard we want to work, and be rewarded (or suffer consequences) accordingly.