When I'm getting ready to putt, I always try to remember the best putts I've ever made. Or I imagine it's match play, and I've got this birdie putt to beat one of my teammates from college who I hated losing to. I get so focused, and it helps me make a nice, aggressive stroke.
If you're nervous over putts, don't fight it. Embrace it. I've had a lot of three-footers where I was shaking. But you've got to realize, it's OK to be nervous. Make it exciting, and look forward to those situations. And if you miss? Who cares what other people think.