Pace of the Greens Drill:

This is a great pace drill in case you are on a busy putting green and all of
the holes are taken. Place three balls at three different distances and try to
putt them so that they finish right on the collar of the green. This drill will
help you to focus on pace rather than steering the putt into the cup
Extend to the Hole Drill:

This is a drill that I recommend for someone who is having trouble making
short putts. First set the ball up three feet from the hole. Now without
taking a back swing "drive" the ball towards the cup keeping the putter low
to the ground. I see a lot of amateurs miss putts because of a stroke that is
more of a jab than a pendulum; this drill will help develop a proper follow
through to the cup.
Drop the Putter Drill:

For someone looking to work on keeping their putting stroke low to the
ground, this drill will help. The drill is accomplished by taking a normal
stroke and at the finish of the stroke drop your putter down to the ground.
After you have finished this drill you can check to see if the putter is on
line to the cup and also check if the putter face is square.