I love the write on cards from Custom Cards (full review here) and use them consistently.  You can use either pencil or permanent marker on them without any problem.  After experimenting with both, I find that I prefer using the Bic Permanent Marker (full review here), primarily because it is a full size pen that clips to my shirt pocket, making it easy to retrieve.

The challenge with all write on cards and permanent marker is getting them clean and ready for the next use.  Alcohol containing liquids like (yecch) Axe body spray, sunblock spray, or even nail polish remover all work well, but I have found they damage the surface of the card over time.

Enter the Staedtler Mars Plastic eraser, $4 on Amazon.  This eraser works well on both pencil and permanent marker.  In the case of permanent marker, you need to use it right after the match, as it doesn’t work nearly as well if the ink is allowed to set in for an extended period of time.  In this case, back to the sprays.  It does require a little more elbow grease than an alcohol-based spray, but on the other hand, your gear bag doesn’t end up smelling like a 14-year old boy.

NOTE: The product on Amazon is slightly different to the photo below.  The product on Amazon is all white, like the left side of the “combi” eraser below that I bought at an art supply store.  The blue section contains a small amount of solvent in the material.  In my tests, the white section was just as effective as the blue section.