The key is getting your putter's face aligned properly.
Here are some tricks. After you've lined up your putt and see
the intended line, place the ball down so that the logo points
straight down the line -- don't point the logo at the hole
(unless it's a perfectly straight putt); point it at your line.
Then, simply align your putter head so that it is perpendicular
to the logo.

Another trick is to pick out an intermediate target: a
discoloration in the green, a repaired ball mark or a particular
blade of grass, that lies directly on your intended line, but only
two or three feet in front of your ball. When you are aiming your
putter head, forget about your line and the hole and focus on this
intermediate target.

Perhaps the easiest way to align your putter head correctly, is to
stand behind the ball and look at your line-- then carefully place
the putter head behind the ball aiming towards your target line.
Then, making sure not to move the putter head, walk around to the
side of the ball and take your putting posture.

This golf tip courtesy of Free-Golf-Tip.com