In reply to Rich2002:
The Climb Skin file is an excellent tool and for under a fiver well worth it.
Small and discrete with a beautiful hardwood curved blade and emery on the back for nails and just the perfect grade of sandpaper on the underside (concave side) which eats through calluses and you get new bits with it which you can stick on. The power you get through the curved shape is what makes this tool great, its width is pretty much the width of a finger segment.
Since I got this I have binned all the bits of tatty emery paper in my bouldering bag for good.