Mavala Emery Boards are particularly recommended for fragile nails: they are supple and have a rough side (yellow) and a finer side (white).
How to use:
Using long, sweeping strokes from side to centre, lightly shape the nail with the rough, abrasive side of the nail file. Then, using the finer, lighter side, smooth and bevel the nail edge. Avoid filing deep into the corners, as this tends to weaken the nails.