Many golfers are not good at judging what effect variations in grass, terrain, and moisture,
will have on a putted ball, but the single biggest reason for the difficulties, experienced by
most people in putting, is a fear of missing a shot which they feel they should be capable
of making. This fear can cause them to experiment with their positions and mechanics in
hopes of turning themselves into a "putting machine". (Weight on the left leg; Feet square
to the target line; Hips and shoulders square to the feet; Shoulders level; Dominant eye
directly over the ball; Left elbow pointed to target; Wrists slightly arched; Thumbs on top of
shaft; Shaft vertical to, or pressed forward of the ball; Etc.)
As soon as you miss a couple putts which you felt you should have made, the tendency to
tinker arises again. Out comes the screwdrivers, wrenches and other paraphenalia used by
machinists in hopes that one of the adjustments will unlock some magical ability. When the
mechanical bit fails you, as it surely must, the whole thing gets sent back up to engineering
for a new "foolproof" design.
The chances of this working are slim at best. Nobody makes them all. The best way to become
a better putter is by simply sticking to a single style until you're comfortable with it. Should you
experience a slump in your putting, head to the practice green for the purpose of reestablishing
confidence in your stroke, rather than changing it.
In this way, whether you're successful on any specific shot or not, you'll at least have the
advantage of not questioning your mechanics.
Consequently, you'll have a better chance to produce the proper speed, and direction, necessary
for each specific shot. All of the game's great players, in their prime, displayed a marked similarity
between their style of putting and that which they played their full shots, and I believe that should
be your aim also. By keeping your smaller swings as similar as possible to your full ones you'll be
trying to produce basically the same motion for every single swing. The only true variations then
become the precise amount of energy necessary for that specific shot, and the swing size through
which you can most efficiently both generate and direct that energy.
Work with it for a while - I promise you positive results. 
