How can I prevent accidental in-app purchases?

Now that Apple requires a password for each purchase, the instance of accidental purchases has significantly declined.  That said, if you're still concerned about potential accidental purchases, you can find instructions for disabling in-app purchases here:http://support.apple.com/kb/HT4213

On the iPad/iPhone/iPod touch devices, the restrictions preferences can be found under Settings > General > Restrictions > Allowed Content (In-App Purchases) OFF.