You can easily change your Google Play Store settings to prevent accidental in-app purchases.

Perform the following steps:

1. Open Google Play Store.
2. Tap the menu button, then "Settings".
3. Under "User Controls", look for an option along the lines of "Require authentication for purchases". (phrasing may vary on different Android devices)
4. You may have the option between requiring authentication on all purchases, or every 30 minutes.
5. Confirm your password on the next screen.

Your device will prompt you for your password on subsequent in-app purchases. As an added bonus, this setting is an extra layer of security against unauthorized transactions!