Every putt you make should always be a straight one. Okay, so perhaps your shot will not end up going in a straight direction, but that's not the point I am making here.
By saying that each putt should be a straight putt, I mean that you should stroke every putt straight down the target line.
In order to hit a straight putt, visualize a straight line that travels from the back of the ball directly down your target line. This straight line should stretch all the way to the hole or least to a point in which you selected so that the ball will break to the hole.
To better increase your chance of success, choose a spot on the target line to aim at. Remember, the target doesn't have to necessarily be the actual hole. It can be anything, so long as it allows you to get your ball rolling in the proper direction toward the hole.