To make a slim jim, find a thin strip of strong, thin,
plastic banding. Fold it in two, and force the loop end down through the
rubber seal between the door and the
window. Feel around until you find the door button release. Fit the loop over the top, and gently manoevre upwards to release the
locking mechanism. This method can also be used to open windows secured by
ram-and-pin devices, and to retrieve keys left in the lock on the outside of the door.
An alternative to the slim jim is the credit-card method, whereby a credit-card is pushed down the rubber seal in order to open the window slightly, and allow a wire with a loop on the end to be lowered into the car to raise the lock.
Note that the slim jim only works on cars made before 1995. Since then, manufacturers have boxed in the door button release.