There are 6 important elements in preventing flat snickerdoodles: 1. Most importantly, make sure your leaveners are fresh!  Both the baking soda and cream of tartar need to be fresh, otherwise, you are doomed for flat cookies.  To test the cream of tartar, add a pinch to a bowl of hot water. If the water slightly fizzes, then you’re good to go, if it stays flat, then it needs to be replaced.  To test the baking soda, add a pinch to a bowl, followed by a splash of vinegar. If it doesn’t fizz, your baking soda needs to be replaced. 2. Chill the dough for at least 1 hour.  3. Bake the cookies immediately after rolling, or place the rolled balls back in the refrigerator to chill again. 4. Don’t overbake.  5. Calibrate your oven.  If your oven is not calibrated correctly, it might be baking at a lower temperature than intended. Try increasing your temperature by 25 degrees which will help set the snickerdoodles before they spread too far.  6. Lastly, if there isn't enough flour to hold the butter as it melts, the cookies will over-spread. So, if you’ve followed the above steps and your cookies are still spreading, try adding an extra 2 tablespoons of flour to the cookie dough.