http://www.amazon.co.uk/Neodymium-Magnetic-Magic-Puzzle-Pieces/dp/B004FGIY6W/ref=sr_1_1?ie=UTF8&qid=1346460658&sr=8-1
Works for me most times - leaves a spread of magnetic spheres to allow a line of best fit to be drawn. Clean and efficient, and then the entertainment of getting them back in the box