The photo at right comes from the Royal Liverpool at Hoylake's history page. It shows Bobby Jones during his Open win there in 1930, and it shows exactly what Jones said in the article I read called On the Proper Method of Shifting Weight. Note that when Jones talks about the left leg, he means your lead leg. (So you lefties can mentally insert 'right leg.')
If we but examine the styles of different golfers, even with the naked eye, it is easy enough to tell whether the weight transference has been a sway or a shift. One characteristic of the proper body action, that is to say, the shift, is that the left leg is straight at and after impact. If you want to know why this is, you have only to look at the time, which marks the left side of the body. It has been lengthened, without lifting the head, by holding the shoulder back while the left hip goes forward. The characteristic of the sway, located again in the left leg, is a decided bend of the left knee in this same area. The entire weight of the body has been thrown forward. The shoulders coming forward also prevent the straightening of the left leg, and either the knee bends more or the player fall flat upon his face. [p94]I find this very interesting simply because Jones is writing around 1930 or so, yet he is saying the most obvious way to get a proper weight shift is to drive upward with your lead leg at impact so your lead hip and upper body don't move too far forward. Push up to move forward in your downswing properly. Sounds rather modern, doesn't it?
To drive upward on the downswing -- that is, straighten your lead knee --it follows that your lead knee has to bend on the backswing. What these two moves do is allow your hips and lower body to move freely during your swing without your upper body lurching around.
Now, how do you make sure you drive upward and don't lean forward? Jones says you "hold the shoulder back while the left hip goes forward." He means that your lead shoulder turns toward your lead heel, and that makes your weight shift properly. You aren't leaning backwards, folks -- you're just turning your upper body so your chest faces the target. (Take a good look at that photo!) This is a strong balanced move into the ball.
If you can't make this move without lurching forward, then you're trying to drive your legs too hard during your downswing. But once you get the hang of this balanced swing, you'll make better contact more often. Good swing mechanics never change...
We just forget how to keep our balance when we try too hard. ;-)