Doing an explosive exercise prior to lifting fires up your CNS and gets your body physically ready to lift hard and heavy. Olympic lifting variations are often used, but here's one that almost anyone can do: the medball thruster. This exercise works great before squatting.
Make sure you drive your knees out as you squat down. Utilize the stretch reflex (the bounce you feel at the bottom of the squat) to rocket your body back into a standing position. Once you reach this standing position, continue to explode up while forcefully extending the arms.