Additional suggestions for added safety:
If possible, attach a second line to the harness (or bosun chair). I use a spinnaker halyard. Use a bowline and not a shackle. As you go up the mast, have your helper take up the slack on the safety line. Do this every 5' or so.
A second suggestion is to wear an offshore harness in addition to the bosun chair (or climbing harness). Wrap the tether 1 or 2 times around the mast, moving it above the spreaders when necessary. This way, if your primary or secondary line parts, you'll fall, but stay with the mast. The drag of the tether will slow or stop the fall.
I know that this sounds complicated or like overkill, but I never worry when going aloft, having hedged my bets. I've used this technique pier-side and while underway (you haven't lived until you witnessed dusk during a race from 55' up!).
Sabre 38 "Victoria"