You may have the smoothest putting stroke in the world, but if your setup is faulty, the ball just isn't going into the hole. Use the following quick, two-part check to build a solid putting foundation every time.
Forearms: Your forearms should be level with each other at address. There are two ways to check this. If there's someone else on the putting green, have him stand behind you along the line of your putt at address and look at your forearms. Your right forearm should hide your left forearm. If you're alone, take your address position and look down at your forearms -- if they're level with one another, you're good to go.
Eyes: At address, your eyes should be directly over your ball and your line of putt. This is crucial -- even if your forearms are correct, if your eyes are too far outside the ball or too far inside the ball, a consistent path will be almost impossible. If you're not sure whether your eyes are over the line at address, check their position by dropping a ball from the bridge of your nose. If your eyes are too far forward, the ball will land outside your ball on the ground. If your eyes are too far inside the line of your putt, the dropped ball will land inside the ball on the ground.