Since Rory has seen such a dramatic change in his putting by working with teacher Phil Kenyon, I figured some of you might be interested to hear what sort of things he teaches. Here's a video he posted in the last month about how to improve your putting rhythm.
The main point Phil is making here is that, when you make a putting stroke, he believes your backswing takes roughly twice as long as your downswing. And his drill is simply to count "one" on your backswing and "two" on your backswing.
BUT PLEASE NOTE that the "one" is at the END of your backswing and the "two" is at the END of your downswing, which makes your downswing twice as long as your backswing during the count. That will create the swing rhythm he wants you to have.
This is only one of the videos that he has up for Odyssey. I picked it because I think rhythm is probably the most important thing to think about when you putt, since this helps your distance control be more consistent. But you can find his other Odyssey putting videos at this link for Harold Swash Putting on YouTube if you're interested.