When "Power Parking" your car, try and keep parallel to the the parked cars, gain some speed, tap the handbrake (Square button) and swing your car in between two cars. When you're stationary, do not press the accelerator (R2 trigger) or brakes (L2 trigger) and wait for the camera to centre onto the rear of your vehicle. You should have a power-parking prompt come up on your screen, and a lovely silver trophy pop.