Tips for a steady arch?

Try putting left hand low (assuming you're a righty).  I used to struggle with a good consistent stroke, until I switched the grip.  Now I just rock my shoulders and watch the ball roll true.  If I miss a putt it's usually a misread. 

Putting is all feel IMO and I think you just have to practice.  Either on the practice green or on a good patch of carpet.  Practice different grips and with different putters and see what works.