So this might or might not work so well with some of your budding litigators/negotiators, but we do something like this (inspired by my mother's parenting):
No allowances or birthday/Christmas money. All your necessities are paid for (this includes certain extras that your parents think are necessities, like learning an instrument). Want items or experiences need to be justified. Whining in checkout lines is not rewarded. Requesting in checkout lines (for example) triggers a series of questions:
1. Are you really going to use this?
2. For how long? Will you still be using/enjoying it a day from now? A week? A month? A year?
3. What future opportunity(ies) are you willing to sacrifice for the sake of obtaining this now? What cost in time, money, or other finite resource is this worth to you?
4. Will obtaining this create undue inequity among your siblings? If so, do you have a proposed solution (that must, of course, meet all the other criteria as well)?
If you can answer all of these questions to parents' consensus satisfaction, and it fits reasonably into the family budget, then yes.
Although I must add that I do throw in occasional unrequested small treats, because nearly everyone enjoys a little surprise. And, of course, perishable items don't usually need to be justified a year out.
