The Maximum Stroke Drill
Pick a specific hole, take six balls and hit them to the hole from a determined distance. If you are more than twenty feet from the hole, give yourself a maximum of twelve strokes total for all six balls. If you can accomplish that, you are averaging the ideal two strokes per green. This helps you learn how to avoid three-putting. You may want to space your balls a foot apart so they don't all have the same line to the hole. That will also minimize the chance of your balls colliding. Don't move on to another hole until you have gotten all six balls into the hole with twelve strokes or less.
Difficult Lie Challenge
Set up your balls in a spot where the green breaks severely left or right from the hole and with the balls at least twenty feet from it. You will see that this is a place where you would never want your ball to land during a round of golf. Even if you putt a ball and it goes right toward the hole, a miss can actually leave the ball farther from the hole than when you started. A steep enough slope will cause your ball to do that. It is fun to learn how to avoid that by not aiming for the hole, but for a spot that gives you the greatest chance of sinking your second putt. Try this with six balls and start by seeing if you can sink all six within fifteen strokes.
Six Balls From Six Feet in Six Strokes
This is a fun way to mentally condition yourself. Golfers blow these close putts due to nervousness. Unless you have a very steeply sloping break or the balls lie on an upward slope from the ball, you will sink the majority of your balls in the hole in one stroke. Challenge yourself by attempting to sink all six balls in six strokes and don't move onto the next hole until you've done it.