Lansky Blade Medic: http://www.amazon.co.uk/s/ref=nb_sb_noss_2/277-5273307-1271315?url=search-alias%3Daps&field-keywords=lansky+blade+medic
There are "better sharpeners but they:
a) cost more
and
b) require care/technique to use
This one just works (and is handy pocket size too).
Cheers,
Mark C