When I am preparing to putt, I choose my speed first, and pick the line second.
In the picture below, you can see what an impact different speeds have on each of the three putts.
All three putts started on the same line. However, the softest putt (red line) broke too soon, and the putt that was struck the hardest (purple line) did not break soon enough.
Next time you are preparing to putt, remember this:
If you want the ball to creep over the edge of the cup like a timid turtle, you need to play more break. If you want to run the ball into the hole like a rabid rabbit, you need to play less break.
Have fun putting with correct speed!
GB
P.S. My three-year-old, Nathan, gave this post his stamp of approval for obvious reasons. :)