Method 1

Put the keys to your handcuffs in a plastic bottle with water. Freeze the bottle. You won't be able to get the keys until the ice has melt, even if the bottle is placed within the reach of your bound hands. You can speed the process up by melting the ice with your hands, though.

Method 2

Handcuff key frozen in an ice cube. The key cannot be used until the ice has melted away. For this to work correctly, a small string or wire must be inserted in the ice cube tray before freezing, making sure the string or wire does not also retain the key. This wire will then be used to hang the ice cube out of reach so you can't speed up the natural thaw of the ice. The main drawback of this method is a single handcuff key can get lost too easily.