Purchase a pair of handcuffs that have two keys. Keep one key and provide the other key to your assistant. Your assistant will hold on to the key in case of an emergency while you attempt to escape.
Choose your location for performing the trick carefully. The location should seem devoid of hiding places at first, but it needs an inconspicuous spot to hide a small key (such as a loose brick that hides a small compartment or a thick lampshade that will conceal a key). Hide your key in the room.
Practice inconspicuously retrieving your key from its hiding spot. You can make the retrieval look like an accident (for example, you could feign a stumble against the hiding space and deftly retrieve the key).
Stretch your wrists to improve dexterity while you wear the handcuffs. Retrieving the key without detection is a hard task, but it means nothing if you can't use the key while wearing the cuffs. Rotate your wrists and practice unlocking the cuffs while you wear them.
Ask your assistant to witness your attempt to remove the handcuffs. Your assistant will tell you if she noticed you retrieving the key so you can adjust your performance plan.