The thing that I use the most is alignment rods putting them on either side of the ball and practicing the path of the putter. I also use a mirror that has a line in the middle of it and that also helps me with path.

I use the Scotty putting guides because they fit in the bag easily and are the same height and width as my putterhead. 

I have the Scotty Cameron putting alignment tool.  It helps me keep a steady path back and through the ball.  My theory, if I can keep the face from moving around, I'll make solid impact.  The rest is just practice and feel.  I also just picked up the Scotty Cameron putting discs.  Not that they are really any better than putting to an actual hole, but I couldn't resist!

Ever tried the CS2?

Really like that you can use it in different ways

Cali line.

I like the pelz putting tutor...many on tour use it as well...

I have an Eclipse Putter Ball, which is great for training a square strike.  Not sure if you can still get them though

Get a metal yardstick. Put a ball in the hole and putt down the ruler - a lot harder than it looks!

Also try a 6" piece of PVC pipe of a Coke can. Try to get them to roll straight.