Putting tips
String line
A great way to practice putting is to use a string line to aid in alignment. Simply tie the ends of a ten foot piece of thin string to the tips of two large 5 1/2 " spike nails and secure with a dab of glue to keep the line from slipping down nail. Press one nail directly behind hole or slightly left or right for breaking putts and press the other end in at the end of the line for the intended putt.
For breaking putts aim the line on the highest point of the break for which you're playing based on the speed you want to hit the putt. The pace you hit the putt will dictate the line you need to start your putt. I always try to pace my ball to finish 12 inches beyond the cup if it were to miss going in and pick my line accordingly.
In aiming the putter underneath the line you're going to line up your putter blade perpendicular to the line and also check you're shoulders occasionally to make sure they are parallel to the target line by placing the putter along your shoulders. Feet should be close to square and many pros are slightly open in stance at address.
The putter blade should also be traveling directly underneath the string back and through during the entire stroke on all putts under ten feet. The blade will need to swing inside the line for longer putts but will depend on the speed of the greens you're playing. In striking putts if your putter isn't staying under the line adjust your distance to the ball. If the putter is swinging outside the line you may be standing too close to the ball conversely if it is to the inside you may be standing too far from the ball. An incorrectly fit putter will have tremendous bearing on this factor. Make sure your putter is the right loft,lie and length. See your pro if you need help.
Change the position of the aiming string frequently to maintain feel and to avoid damage to the green from stance.