Oh, Pare Me
Sharpeners come in all shape and sizes to deal with the array of liners and sticks of all sizes and sorts out there, so a parer with two sizes in it might be your best bet. You can buy yer bog standard one in the supermarket, in your local chemist or Boots, and you won't pay more than a couple of quid. Good sharpeners for girls on the go are the sort that come with a little cover, so the parings stay safe inside until you can empty it, and not scattered all over the inside of your lovely makeup bag. Tweezerman do a range of these.
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If you want to be all fancy, you could treat yourself to a Shu Uemura one for considerably more. Or what about one from Nars? It's pretty clever - it has a removable adapter which allows you to adjust the size to fit even the largest pencils.