'I've done this thousands of times'
In 2004, I was given the opportunity to hole the winning putt at the Ryder Cup. It was on the last green at Oakland Hills and it was a curling five footer - one horrible distance whatever your handicap because those are expected to go in.
Once I'd agreed with Alister, my caddie, that it was right lip, I walked up to the putt repeating to myself "I've done this thousands and thousands of times" (you can see me mouthing it on the video).
I take no more than four seconds to hit a putt, any longer and doubt can creep in. I didn't think about anything at Oakland Hills other than repeating that mantra. It can help you, too.
You have at least 10 thoughts when you prepare to putt (is it uphill, does it break, how fast is it etc) and if one of those is negative, then you've already missed it. Langer used to take 11 seconds to hit a putt, but I think that's too long.
The worst thing amateurs do is watch the clubhead go back. You can't stay still on a putt if you watch the clubhead. No one stays motionless over a putt as well as Tiger Woods - you should copy him.