If you don't already have one (which IMO all kids should have one) you should get a savings account and put all the money you make or recieve in it, that way it's not sitting in a shoe box or a hidden somewhere in your room for you to spend.
Everyone has given good advice thus far, get a paper route, cut grass, rake, shovel snow, and all that good stuff. Incentive for getting good grades is also good, talk to your parents and grand parents about some sort of grade = cash (quarterly or semesterly).
Here are some other ideas.
See if your parents (if you don't or haven't already) will give you a weekly allowence for doing certain chores around the house. Keeps your house clean and looking nice and you'll get paid.
Talk to some people in your neighborhood, see if you can make some money by walking peoples dogs. Seems like a lot of people love having dogs, but don't walk them and people will pay for this.
On top of walking their dogs, it's still warm out, how about washing peoples dogs?
In the vein of washing things, how about washing cars? You could arrange this with your nighbors or get together with a bunch of buddies, pick a spot and make some signs... split the cash.
I know this may seem very like the dark side, but if you also play guitar sit out and pan handle. I've done this in Madison before, I was broke for the weekend until Monday, so I sat out for a couple of hours just playing my acoustic and made some money. I've friends who will ride on the train with their guitar and pan handle and make better then min wage.
Collect all of your aluminum cans or scrap and take it in. Possibly ask the people in your neighborhood if you could have their waste cans as well. People are paying high dollar for aluminum right now, so it's actually a very good idea. My band has actually recently started doing this (we practice at my guitrists house and they drink a lot of soda and beer) and we all save ours at our houses. We've raised hundreds already.
Save all of your birthday and holiday money.
Hope this has helped.
-Benny