I "pre-shave" mine. First I take a wood screw and run it through the lanyard hole and into a piece of wood with my drill. I put the wood/attached mag block into the upturned lid of a cardboard box. Then I take the drill and systematically reduce the block into curls and shavings (you have to experiment to see what size drill bit works best for you). When I'm finished I simply take the lid (now full of magnesium shavings and saw dust) and dump it into a plastic container.

I've carried these wood/magnesium shavings in a small ziplock and I've added them to my Vaseline cotton balls...both ways work great.

A variation I've wanted to do would be to cut the mag bar lengthwise, preserving the lanyard hole/striker bar side and then clamp and drill the other half. this would be the best of both worlds: mag shavings and a homemade mini mag bar.