I found this online.
I've always wondered if there was a way to sharpen my seam ripper until I could get to a store for a new one.
(ummm... I mean sharpen my friend's seam ripper....I don't use one because I never make mistakes :wink: ....)

"To sharpen your seam ripper, wrap 000 or 0000 steel wool around a crochet hook or pencil. Holding the seam ripper in your non-dominant hand (left or right) and the pencil or crochet hook in your dominant hand, move the pencil or crochet hook with steel wool wrapped around it in an upward motion. Do this to each side of the "blade" of the rippers. Emory paper will also work."