Show me a long hitter, and I'll show you a good shoulder turn. Not only a good shoulder turn, but one that places the upper body loaded over the right side at the top.
I like to say, "rotate your left shoulder behind the ball." By doing this, you achieve both goals – a big shoulder turn and getting loaded on the right side.
Back in my GolfTEC days, we used a shoulder harness and belt with sensors to measure turns, tilts and bends throughout the golf swing. If someone said that they would like to improve their distance, the first thing we looked at is shoulder turn. In most cases, they would only be 75° or less of turn when 90° is optimal. For most males, for every 1° they would add to the deficit short of 90 they would pick up 1 yard. Yes, 1 yard for every degree. By the same token, you would lose a yard for every degree past 90. Not because it hurt power so much, but "centeredness of contact" was compromised.
L to L Drill
Probably my favorite drill. Turn 90° with the shoulder, and a 90° hinge between shaft and wrist. The combination of a full turned and full wrist hinge will add distance to every shot.