Swinging Too Hard
The most common misconception in golf is that if you swing the club harder, the ball will go further. The truth is that excessive distance and accuracy are achieved with a combination of proper weight shift, maintaining good balance and clubhead speed. Excessive clubhead speed will cause you to lose control over your swing and get results you don't want.
Eye Off the Ball
Another common mistake golfers make is lifting up their head to see where their shot goes before making contact with the ball. Lifting your head before contact creates two unwanted scenarios. The first is disturbance of your hand-eye coordination as you make contact. Keeping your eye on the ball as you make contact helps ensure you make clean contact each time. The other is lifting your upper body and causing you to either miss the ball completely or hit a shot that just trickles along the ground.
Positioning the Feet
In order to hit the ball straight, your feet and shoulders need to be lined up with the target. Both feet need to be in a straight line shoulder-width apart. If either one is back from that straight line, you will either slice or hook the ball. Practice keeping your feet in a straight line by lining them up with a club or other straight object when you take your stance.
The proper golf grip is interlocking the pinky finger of the lower hand with the pointer finger of the upper hand, and then making sure both thumbs are in a straight line pointing down the shaft. The grip should be closed but not tight. Leave some room for the impact of the club and ball. If you leave your grip open, you open the clubface and have accuracy problems. If you grip the club too hard, you may get repeated instances of feeling that bumble bee in your hands when you make contact.
People take club selection for granted, but it is one of the most important parts of the game. Part of good club selection is being able to remain humble even in the face of peer pressure. If everyone else in your group is able to reach that par 3 with an 8-iron, you might feel pressured into using an 8 when you know a 7-iron will do the job. Don't play other people's game--play your own game and use the right club. The especially applies as you get older. Just because you could hit the 3-wood 230 yards when you were 20 doesn't mean your 50-year-old body can still get that kind of distance. Use the driving range to gauge the current distances of your clubs, and make good club choices on the course.