M.B.
We follow Dave Ramsey's advice & pay our kids "commission" instead of "allowance". As adults, we get paid when we work -- this is the commission concept, so it sets them up to realize they must work to get paid as they get older. Some things they have to do because they are a part of our family & we all need to help out. Other things, we've assigned a commission amount to a chore, and they check it off as they complete it. We do vary the amount based on the difficulty and ability. We also have assigned a maximum amount that each child can earn each week (that way, we can budget & make sure we have enough money to pay them each week!). :) It's basically their age multiplied by 3. For example, my daughter just turned 9. So her maximum amount went from $2.40 (8x3) to $2.70 (9x3).
I'd suggest you figure out how much you guys are willing to pay each week. Then, make a list of chores they can earn commission for doing & assign dollar amounts for each. Make it clear that certain chores are just for being part of the family (clearing dishes after a meal, doing homework, picking out their clothes each day, etc.). Encourage them along the way! Even with the small amounts my kids get, they've saved up & been able to purchase a lot of things!
We always follow the give/save/spend method, where they "give" 10% first thing. For us, they give to something specific at our church. If you don't attend a church, you can give to a local food pantry, or the library, or something that is important to your family. Then they save 70% - 20% towards their long term goal (Nintendo 3ds for my kids) and 50% towards smaller goals (movies, toys, etc.). And the final 20% goes in their "spend" envelope. This can be a spur of the moment choice (a book, small toy, fingernail polish...something they aren't "saving" specifically for). It works really well & sets them up to handle finances as an adult.
Let me know if you have any specific questions. We enjoy this! =)