When most golfers face a really long putt, they get so focused on hitting the ball hard that they rush the backstroke and make a short, stabby motion. The result is poor contact and a putt left short.
On those 40- and 50-foot lags, you should do just the opposite: Make a longer, slower stroke. You have to give the putterhead time to gain momentum on the downstroke, which starts with making a longer backstroke. Swing the putterhead back behind your hands, letting your wrists react to the weight of the club. Your left wrist (for righties with traditional grips) starts in a cupped position but should flatten on the backstroke (see photo).
On the downstroke, you shouldn't have to think about adding speed. By flattening your left wrist going back, you've created a little hinge: Use that hinge by hitting with your right hand and letting your right wrist flatten through the strike. Allow the putterface to rotate closed. Many golfers try to hold the face square, but it will close.
Remember, distance control is your focus on lag putts. To get the right pace, make a long, slow stroke -- and don't be afraid to allow a little wrist action.
If you're like a lot of amateurs, you peek on short putts. Maybe it's because you can see the hole in your peripheral vision, or you feel anxiety on putts you think you're supposed to make.
Whatever the reason, here's a great drill for breaking the peeking habit. Pick a four-foot putt on the practice green, then stick a tee all the way down in the ground (or put down a dime) and putt balls right off the top of it. Now here's the key: Make sure you see the tee after you've hit the ball. This will help you to keep your head in place and your putter on line.