I use a credit card in my teeth. Line it up so the edge runs in your eye-line along the target path. This gives the line and keeps your head still. They make hats with lasers (that sharks wear) to do the same thing...this is the economy version.

Get a pendulum. Find your tempo for a normal stroke. One tick in the back swing, and one in the front. Let your arms hang without tension, and let your hands stay loose on the grip while locking the wrists. Set the pendulum behind you so you don't see it, and only hear it. Make all different length putts to no particular spot other then the distance. This engrains the idea that your tempo is the same for all lengths. Never deviate from that and you will increase your short length make % and your long lag %.

When there are a lot of people putting to 1 hole, and you are getting frustrated waiting. Make you own hole with a Tee or a quarter.

Practice breaking putts with a piece of chalk. Start at about 1 foot. Let the ball die into the hole. Make a small mark at the point you putted out to the point of your line...(say 2 balls outside the hole). Then step back 4 feet. Aim out about 1/2 of that distance...now about 1 ball outside that 1-foot mark (all depending on the difference in the speed of the greens). Once you get it to die into the hole again, make your mark (where you putted it again) out to the point where you had to aim it (1 ball outside where you just putted it), like you did for the 1-foot putt. Keep doing this until you get to any distance. Remember to 1/2 the distance you had to putt out to from the previous putt. This works on a single breaking putt only. To get multiple breaks, you have to start thinking about the speed of the first break. It will not take very much. Work from the hole back like you did for the 1 break putt, but when you reach the second break, you have to start over from that point. Apply the chalk to that point of the last break and play the ball out to the other side your desired length remembering that the speed is going to cause the ball to break very little. You now have a lot of points in your vision line to see how the ball breaks. Make a lot of putts to your first point over and over and over again until you start to see some interesting results. That speed is everything. You can hit that spot pretty easily, but unless you have the perfect speed, the ball falls short under the hole or blows past above the hole. Now realize which set of misses you would want to have to finish up your 2 putt with...the one under the hole, right? It will always be up hill. So when lagging a 40' long double breaker, error on the short length always, and you will have an easier 2 putt.