2 Ball Impact Drill - from the 2010 US Open
Are you square at impact on your longer putts?
- Put 2 balls side-by-side
- Hit a 25 foot putt hitting both balls at once
If one ball goes considerably further than the other, then the toe or the heel is hitting it prior to hitting the second ball... not good.
The Hank Haney Putting Impact System was designed exactly to help this error.