Is your piano teacher getting on your case about playing piano with flat fingers? Check out these tips courtesy of my fellow teachers (and friends), Katrina Thompson Fost and Melodie Archer: Finger Posture Tip #1: The Water Bottle Trick
If you squeeze the bottle too hard, you will have too much tension in your hand and forearm (not to mention, you'll get wet). This works well with an egg. Be brave and try it with a raw egg (you'll learn really quickly!).
Finger Posture Tip #2: The O's Game
This is an extension of the "Making O's" or "Making Eyeglasses" warm-up in the Piano Adventures books. Make an "O" and gently squeeze your finger and thumb together:
Then, test how firm your finger is pushing into your thumb by pushing on the curved finger - anywhere from the nail to the first knuckle is fine. If that finger collapses (goes flat), then you need to firm up your fingertips a little bit more.
You could even get a family member or friend to push on your curved finger. Go ahead a turn it into a bit of a game.