Check Your Stance
Golfers tend to get sloppy with their stance when they need to make a big shot. For example, you could be tied in your match on the 18th hole and in need of a big drive to get over the bunker down the right side of the fairway. In order to get a little extra power, you angle your left foot in order to promote a fuller swing. That's fine and it makes sense. However, if you open up your shoulder as well you won't get the distance nor will your shot go straight. Make sure your shoulder is facing your target and does not open up.
Drive with Your Hips
The hips need to be the engine driving your swing. Everything else must follow. That includes the knees, the legs, the hands and shoulders. As soon as anything else starts to go at a faster speed than your hips -- usually your hands -- that's trouble on a golf swing. If you want to swing harder, drive your hips through the ball and let your hands follow.
When your golf game is in its crucial moment, that's when you tend to forget your fundamentals. If you go to the fourth tee after a good start to your match, you can probably keep your head down as you follow through and your hands will finish nice and high. However, if you need a big fairway shot on the 17th hole or a huge drive on the final hole, will you keep your head down and will you finish your shot or will you pick your head up or stop your swing in order to admire your shot. Finish your shot with your hands at shoulder height.