To keep the pick from sliding back lock the back of it into the groove between the tip of your index finger and the middle part of the finger. Choke up on the pick leaving the smallest tip possible while keeping it locked in that groove. While remaining in a natural playing position use your right forearm to apply pressure on the face of the body (were your arm would be anyways.) It should feel somewhat natural. Use your pinky finger to anchor on the pick gaurd, and if you need to use your ring finger too. Play some songs with alot of palm muting like this. You will have mad control.