Putting in the time to practice is going to be what helps you improve. Be patient and confident and treat every putt individually. I was most successful staying within my comfort zone while developing my putt. It kept me confident and allowed me to focus on making the next one. I would step outside my confidence circle every once in a while to shake things up though. It helped me forget the missed putt and have the confidence to keep trying.
This is really it here. Just getting out there and doing it. Mark Ellis, in his putting clinics, has several important bits for everyone to remember. And while I don't agree that all of them are the best for everyone, I do think that his 15-minutes at a time idea is good. Two 15min putting sessions will help you tons, heck, even one 15min session a day will improve things a lot.