ON THE HEELS: Weight back causes thin shots.
Proper weight distribution is crucial to maintaining good balance and posture throughout the swing. When I see players struggling to make solid contact, one of the first things I check is where they set their weight at address.
Your weight should be 50-50 on your left and right legs and between the balls of your feet (those cushioned pads just below your toes) and your heels. I'm surprised how many people think their weight should be supported by their toes. Starting with your weight too far forward is one of the worst mistakes you can make. It throws off all of the critical angles--spine, legs, hips, shoulders and head. That creates a domino effect, resulting in a weak swing and fat shots. Setting your weight on your heels is not as bad as leaning over too far, but it still causes problems, namely thin contact.
Here's an exercise for checking your weight distribution: If your weight is set properly at address, you can't be easily pushed off balance--either forward or backward.
Watch the pros. They shuffle their feet around at address to find a balanced position. And they don't swing until their feet are set. Neither should you.