The Golf Blogger posted today about the "Bowen Ball Swing Aid." The BBSA costs "only" $39.95 and is nothing more than a lucite stick. We're economical (that's slang for "not completely stupid"), so we advocate creating your own training aid: glue two dowel rods together (one has to fit into that hole in your grip), fold a coat hanger appropriately, etc. Heck, tape a pencil to the top of your grip if you want.
The images on the BBSA "How it Works" page explain the functionality pretty well: one end of the rod goes on the butt of the club, the other leans out perpendicular to the shaft and parallel to the clubface. Take a normal grip and swing, but don't let the aid touch your arms. If your swing is overly wristy or gets off-plane near the top of your backswing (laid off or across the line), the coat hanger/dowel/expensive piece of plastic will let you know. You'll, quite literally, feel it.
Now that you've saved roughly $39.67, well, buy yourself a dozen good golf balls.