In this video, Bill Rogers (Rogers Shooting School) demonstrates a new training aid just added to his Rogers product line: the Tap & Rack Training Aid (TRT). This small insert chambers into a 9mm or .40 cal magazine and inhibits the slide lock from catching, thereby allowing the user to safely practice "dry fires" without ejecting dummy rounds or alteration to the magazine.
http://www.youtube.com/watch?v=lz8KEt9nHPg