Step 1: Stop Spoiling Your Kids!
Then expect them to stick with their choices
Once they make a decision about the cereal, the candy bar or whatever you've put in their universe, don't buy another one until it's gone. If you head back to the grocery store the following week with a full box in the pantry, you can explain to your kids that you don't need a box of popsicles because you (and they) haven't finished the box of Fudgesicles they picked out last week. Don't succumb to the whining. Just keep your cart moving through the aisles.
Now that you're ready to stop spoiling your kids, it's time to teach them more responsibility.
Start step 2: How and when to give your child an allowance
Get all the steps to raising money-savvy kids