I use a line that came with the Top Climber for this sole use. I attach it with a bowline to the main halyard that is also tied in a bowline. I trust bowlines more than shackles or other knots.
I use a one-way knot on a second halyard, manually sliding it up and down as I go. It's tied to the TopClimber and also under my arms. The under-arms part won't help me live, but it will help me not fall, I do keep the line going to the one-way knot short so if I am hanging under the arms as part of plan B, that I could possibly decend.
I also keep a separate line that I wrap around the mast a couple times, and untie it to get by the spreaders. I tie it to things when I'm working in one spot.
I go up with absolutely no one around, without reservation. Those are enough backups for me.
The most dangerous part is decending the last 10 feet. You forget that there are a lot of hard things to hit your knees on. You'll see. Just come in for a landing slowly.
Check the seams before you go up each time.
Wouldn't be without a TopClimber. See this thread:
And add to it with some pictures when you get back down!
P.S. No discussion of climbing the mast is complete without referencing the ultimate "going aloft" video. Watch this to get mentally prepared!