My one word: BELIEVE
I remember watching a PGA broadcast a while ago, where they mentioned how Vijay Singh had been putting really well. They referred to the fact that he had adopted a method of saying to himself, "I'm one of the best putters in the world." I remember thinking how powerful that was. True? At the time, it seemed so, but historically, Vijay was erratic. I comes down to, what is the alternative, "I'm marginal"?
I adopted it to my putting in this way: when you read a putt, there's really only two things you can say, "I've read this right, I've got the speed, it's going in," or "something's off, it's going to miss." The first won't always be true, of course, but if you thought the second, why bother? The bottom line is, if you're going to bother, then you must BELIEVE the first.
I find this easier to do on putts because you're not worried about push cuts, smother hooks or water hazards on putts, it's just rolling a ball. That strikes me as more achievable on an given shot. My swing has been in an overhaul period, but I can tell you that, when I believe on those shots, good things happen, too.
So, I'd say the one word is BELIEVE.