The training aid that we use most with our students is the putting mirror by Eyeline Golf.    This product is very simple and extremely effective in helping a player continually work on the correct fundamentals.  The putting mirror promotes good fundamentals and good fundamentals promote lower scores.  The mirror has lines to help you position your eyes correctly, work on proper alignments including putter face and shoulders, and helps to insure proper ball position.  This way you are always practicing the correct fundamentals.  
The mirror comes in two sizes.  The larger has the capability of adding putting guides and putting rails, and the smaller one fits in your bag.   This is the one product that can benefit  every golfer of every ability.
To order the EyeLine Putting Alignment Mirror click on the button