Method 1 (Partner): Do not let the handcuffs close all the way on your wrist instead when you are being handcuffs push your arm towards the person cuffing you. So you will have extra room to wriggle out when you slide them down.
Method 2 (Shim): You can make a shim from an everyday object such as the metal part on some pens (pen clip). Place it in between the silver teeth and the black locking mechanism of the cuffs. Push it in and they unlock and you are free.
Method 3 (Key): Most handcuffs all take the same key. If you are doing a trick just hide it on you, but that is kind of lame this method is mainly meant for information if your cuffed against your will and don't have a key or have lost it you can just go to a gun shop and buy one.