The shoulders should never drop on the back swing or the down swing. If the shoulder drops, the swing will veer off course when the head of the club makes contact with the ball. Imagine a piece of wood being tied to your shoulders from the back.
Regardless of the grip you choose, ensure that you have a firm grip but do not squeeze the club as hard as you can. A firm grip with some "give" will allow you to have better control of your club and your swing. A loose grip can cause you to change the direction of the club.
It's the natural tendency for the golfer to look up before they complete the swing, because they want to see their shot. This can cause the golfer to lose concentration on the swing. Never look up until the swing is completed.
Rotating your hips too far one way can lead to bad swings. Your hips are essentially the most important body parts used in the swing because they control your upper body, which is crucial to the swing. Rotate them slightly so your downswing is not so jerky.
Your stance is important because you are targeting the spot where you want to hit the ball. Be comfortable, not mechanic. Keep your left foot (right-handed golfers) in line with your left shoulder and the right foot in line with the right shoulder.