First, let's understand that the putter gates as we swing it. My point is this, don't try to make your putter go straight back and straight thru. It just does not work that way. The toe of the putter will swing open on the way back and close as we swing thru it. 

Here is the most important thing to understand. When you set up to putt, if you place the ball too far forward in your stance the tendency is to pull the putt. This is because the toe of the putter had too much time to close and in fact was closed when you contact the ball. (toe-first) The opposite is true as well, if you have the ball too far back of your stance the putt tends to be pushed with a heel-first strike.