When Phil Mickelson turned pro, he used a blade putter that would close to the left when he placed it on the ground at address. Most putters are like that; they rotate to the left because of the way the sole is ground. To get the face square, Phil would forward press before the stroke. But the forward press decreased the loft of the putter. So he switched to a putter with more loft. When Phil changed his stroke by getting rid of the forward press, it meant he didn't need the extra loft anymore, so he switched putters. But the style of putter wasn't for him, so he switched again, to the model he has now. It's a blade putter I made for him, one with the sole milled in such a way that the face doesn't close to the left when you set it down at address. Phil's evolution is very typical. Most of the changes a golfer makes with his stroke are a result of the putter, not your technique.