Try a pair of rolling hitches.
Despite advice to the contrary seen above, I've had good luck getting to the top of my previous two boats' masts barefoot and unassisted. Speed is of the essence as fatigue sets in quickly and it is important to be back at the deck before it is overwhelming. The 'shakey leg' syndrome common to mountain climbing is a good clue you don't have much time left.
A guy I know used an innovative technique, two short lines, tied around the mast with a rolling hitch, then a two foot whip to a bowline with a one foot loop as a foothold. The idea is that you can transfer your weight two one loop at a time, sliding the other upward. A strap of rubber or vinyl attatched to a stout harness might also prove helpful.
People have been climbing up things for a long time. If you are afraid of heights don't try anything I've mentioned. If you aren't and like to take risks, give it a shot. Have a friend around to drive you to the hospital.
Sailing a '74 Challenger 40' Ketch rig out of San Francisco