Wow, lots of great info on this subject. CNOSIL nailed so much great info, I won't add much. Even with all the data & tech available, a very good fitter is still a key to putting tips specific to you, while he's using the technology. Obviously, Reading a green is critical, once I found a putter I absolutely have faith in, all I need to know is, do I hit my ball where I intend to hit it.
I found a 48" ruler, just wider than the golf ball, is my final training aid. If I can pick a distance and hit it straight without it falling off that rail, I know I have my face square, and that is what gets me to my target on the green (and in the hole if I picked the correct line + speed).
For $10 with tax included, HD has a 48" that is 2" wide. Then you can downsize to a ruler 1 1/2" wide to hone in the stroke even more.
If you can't hit it 40" off the end, just keep moving the ball on the ruler until you can master a distance, then challenge yourself to the full length of the ruler. Plus, you can clearly see your putting arc in relation to the ruler being a straight edge, and how far back your stroke is. I use the ruler to ensure I hit thru the ball longer than my backswing to ensure I got my ball rolling end over end.
You can use this indoors with little fanfare for setup with whatever putter or technique you are using.