Something like this works fine. Just a couple strokes and you're good to go. OR, you can just replace the inserts.

http://www.keystonecountrystore.com/Archery_Equipment_-_Supplies/Broadheads_-_Points/Broadhead_Sharpeners/Magnus_All_Sharp_Broadhead_Sharpener_/Page_1/PNT3113208.html