Try to use a sharpener that has a grove in it. I have a little diamond sharpener and it works great its about half the diameter of a pencil and has a grove down the center to run your hook down. Do this about 5 or 6 times and it will be sharp depending on howw dull your hook is. If you damage your tip to much, like on a snag tie a new one on. don't waste your time trying to sharpen it. And I don't believe in dragging your hook on your nail.
Your just dulling it, as you do that your taking away the "sticky point " of the hook.
I would also recomend removing the rear treble hook off your plugs and replacing them with a si wash (of the appropriate size according to the size of plug your using) attatched to a 6 bead chain swivel.
Hope the info helps