Being a scrawny, mechanically and electrically savvy person has raised me to new heights on more than one occasion. Couldn't resist the Bob Hope joke...
Now, the serious business of going up and down a mast in safety:
The first thing you need to do is make sure that the halyard you'll be raised on is in good shape. Sounds simplistic, but when it's your own precious hide hanging from thirty to a hundred and thirty feet up, I'll take simple. Look at the line for fraying, cuts, etc., with a pair of binoculars. If the boat has internal halyards, tie a messenger line on the chosen halyard and raise it to the masthead and check the 'tail' for cuts, abrasion, etc.
The boatswain's chair needs to be comfortable. If it isn't, you'll be squirming around looking for a soft spot. This is experience speaking. My current owner went to West Marine and bought their most expensive, top of the line, ad infinitum chair and let me tell you that after ten minutes in that thing, it was killing my legs. Don't be embarrassed. Pull the thing out of the package at the store and pull it on. Sit down on something and lift where you'll be lifted. If it pulls on your legs in the store, it'll be a hundred times worse when you're up at the top of the mast. While we're talking about the chair, it needs to have a strap that goes between your legs, so if you happen to get turned upside down, you don't necessarily fall out. Back in the bad old days when I raced the Transpac, the seat I had didn't have a strap, and I nearly fell out one dark night while I was retrieving the busted spinnaker halyard. Before I crawled back into bed that night, I put a piece of nylon webbing on the thing--hand sewn with whipping twine.
Next thing: the chair needs to have pockets to carry stuff like tools, bulbs, grease, and so forth. Having a pocket that is part of the chair's sides is worthless, because as soon as your weight is fully on the chair, the pocket is like the front pocket of your teenaged daughter's tightest jeans. Useless, to say the least. Barring having a pocket, you should carry a CLOTH bucket with your stuff in it.
Which brings me to the next thing. Tools need to be tied to you, the chair, or the bucket. Dropping a screwdriver, neednosed pliers or whatever can turn them into a lethal weapon at worst, or something that's going to put a ding in the boat at least. (Or your only set of electrical crimpers will put a ding in someone or the boat, then bounce overboard in deep water...Been there, done that, too.)
For stuff you can't tie on, make sure they're clean and not slimy with grease, oil, water or sweat if you possibly can. Sometimes that doesn't work out, so take two or three of these items up with you.
Carry a messenger line. A piece of cheap 1/4 inch line tied to the chair will allow you to raise up that tool you forgot, a cold drink if you're up there a while, or the camera to get the obligatory picture of your feet and the deck below. It's also useful to carry a digital camera up in the bucket. That way you can take pictures of what you find wrong, fittings, etc. You can, all joking aside, also get some marvelous pictures when you're up forty feet or more...
Now we're down to the important part. Take the chair and go to the mast. If your halyards have snap shackles on them, connect the shackle to the chair's lifting ring, then take that roll of electrician's tape that's in your bucket and tape the pull-pin on the shackle so you can't accidentally snag and pop the shackle open. If your halyards don't have shackles, tie a bowline and then tape the tail to the halyard. Yes, yes, I know. Bowlines don't come out...and if you believe that, I've got a nice lot for sale in Georgia that is only a little swampy. Six inches of electricians tape is cheap life insurance.
Wear sailing gloves. Period. You can grab a shroud and find a meathook, stick yourself with a screwdriver, etc. Once again, cheap insurance.
Have whoever is turning the winch take four turns around the drum. Make sure that the person who is tailing that winch has gloves on. If the line starts to slip, gloves will keep him (or her) from getting a bad rope burn.
You can help the winch operator by making sure you don't get hung up on anything as you're being pulled up. Make it a point to stop at the spreaders and look at them for wear, corrosion, breakage, and so on. It also gives the person on the winch a little break.
Once you arrive at the masthead, relax a moment. You'll be stressed a bit, even if you've been up and down as often as I have in forty-five years of sailing. Inspect the shroud fittings. Look for corrosion. Are the fittings turning gold? You might need to consider a professional look at them to make sure they aren't ready to fail. Look at the turning blocks for the halyards. Make sure the sheaves aren't hogged out on their axles. A little bit of spray lubricant can go a long way up there.
Check the attachment points for spinnaker halyards. Are the shackles in good condition, or nearly worn through? Is the bail or whatever in good condition? Is everything with a nut/bolt tight? How about that VHF antenna? Does it look okay, or is the PL-259 connector for the coax cable a nice shade of green or black? Is the coax itself okay? Tug on it a little. Not hard, just a little. If it feels secure, it probably is, but a coax that is corroded out will pull right out of the connector with almost no effort. If it's bad, you can get a gold plated one at Radio Shack or West Marine. The gold plated ones last much longer and are not prone to corrosion like the cheapies that come with most antennas.
Now you're ready to work on the lights. Oops. You can't reach them sitting in the boatswain's chair. What to do?
This is where you have to raise a second halyard with a couple of loops firmly attached so you can stand up in the loops. If you think you're going to have to do this, try it about a foot off the deck first. Don't be forty or fifty feet up to try something new. This has a second problem. You'll need to be able to do the entire job with only one hand, because you'll be holding on to the mast with the other hand. For myself, I made my own boatswain's chair and it fits my scrawny self very closely. The attachment point is very close to the seat bottom. The only catch with this is that your center of mass is above the point you're attached, so going over backward is a very real possibility. But the bottom line is, with my own rig, I can be raise up high enough to be able to get my hands on most of the lights I've seen mounted on the masthead, the spar-fly, and all of those other goodies that are mounted on top of the mast without having to try and stand up and do the job with one hand.
Anyway, you don't want to spend a lifetime going up and down, so take a little time while you are there and fix things right. Clean the contacts on the lights if you have to take them apart. Change the lens if it needs it. Smear some silicone dielectric grease on things to keep moisture out and away from the electrical contacts.
Finally, test things before you come down. Nothing is quite so irritating as changing out the tri-color light's bulb only to find out it wasn't the bulb but the socket or wiring that was bad. Once again, been there, done that.
Take a last hard look around to make sure you didn't miss something. Take a hard look at where the halyards go over their turning blocks when the sails are down and the halyards are dock-secured.
Okay, it's time to come down. That's accomplished by the person who is tailing the winch. Have them put one hand over the line that's wrapped around the winch while they hold the tail in the other hand. When they allow the tail to approach the winch, you should start down. Four turns around the winch, however, may be too much friction, so they'll have to take off one or maybe even two wraps. That's where the hand around the winch comes in. A firm grip on the line that is wrapped around the winch is all it should take to hold you in place while they VERY CAREFULLY take a turn off the drum. After taking a turn off (3 turns left now) I would expect you to start down when they loosen the line. By keeping the hand around the drum, if they should lose the tail in the other hand, all they have to do to stop you is squeeze hard. It's not rocket science. It just takes focus and attention to detail.
The tailer should make his or her motions smoothly because up in the chair, a jerky motion at the winch is amplified. Someone who is experienced can let you down in one long smooth motion. I prefer to have beginners keep a good hold on the tail and only let me down an arm length at a time.
I know I've been long winded, but I've spent a lot of time in boatswain's chairs. Twice on the Transpac race, once on a LA to Tahiti run, and more times than I care to remember tied up in a marina or at anchor someplace. I even got the ride of my life down in Fort Lauderdale a few years back when one of the superboats had a little messenger line come untied from the company's logo flag, and that flag was flogging the next boat's mast and rigging. All of the 'guy crew' was ashore except the captain. I offered to go up the mast to retrieve the flag for him and he gladly accepted. The mast on that boat was about 120 feet from the water, and I had to go to the masthead to unjam the halyard so we could get the flag down. The view was, to say the least, spectacular. I've also gone up the mast of a Cal 47 while racing, and we never stopped racing. That is a little more spooky, because you look down and there's nothing down there but water.
In any event, I've survived all of these trips because I pay close attention while I'm standing on the deck, and I make sure that whoever is tailing the winch has their act together.
I hope this has helped. Good luck and don't forget the silicone grease...