A simple feel and confidence drill that Mickelson uses.

Find a hole on your green, ideally on a slight slope, and place a dozen balls around the cup 18 inches away from it. Try to hole all the putts around the clock, and while doing this take into account the different breaks. Next, have another trip round the clock, rolling the balls in at different speeds, knocking one in the front door, then another firmly into the back. When you are consistently holing out from this range you can extend the circle out to three or four feet.