For a depth gauge on a drill bit, masking tape works well on the first few holes. But it doesn't take long for the tape to tear or push up along the length of the bit, thus destroying your accuracy.
For a depth gauge that lasts and lasts, I use a short piece of clear plastic tubing (available at the hardware store). The tubing should have the same or slightly smaller inside diameter as the drill bit's diameter. Chuck the drill bit into a portable drill and slip a section of the tubing over the bit. Make sure the tubing is pushed up firmly against the jaws of the chuck. Then, cut the tubing to match the hole depth.
--Craig Holden, Ames, Iowa