[Hamlin]

When my grand dad was still doing cabinetry work, he sharpened most "spade" bits with just a small flat file.

 

[Hamlin]

If you don't have or can't find a file that small, try an emery board or you might try a very fine grade of sand paper, 400-800 grit. Just remember, sand/file very lightly, just enough to remove the spurs. Spade bits, I've noticed spade bits have a tendancy of getting spurs on them. If all you're going to be doing is running wiring, try getting auger bits. They hold up much better than the spade bits. They also "clean-out" much better. (Removal of the wood chips)