Go to the practice putting green before you play a match or a tournament and strike 10 to 20 putts before your begin your round.
Don't stand over the putt a long time and think about your shot. A quick assessment, a practice swing and then hitting the ball will help most golfers produce their best putt. Standing over the ball may induce the golfer to develop an involuntary case of the yips (tremors in the hands before striking the putt). This is especially true for left-handers who are new to the game who think they have to do it "differently" than right-handers. They don't. They just have to strike the ball.