I'm pretty sure that it isn't entirely true that when someone has passed out they already have brain-damage, but it is true that as soon as they go, you need to release them. It *is* best to avoid gettign them to the point of passing out if you can.

I certainly endorse only using the hands, for safety and aesthetic reasons. Using a bag is dangerous because they can be tricky to get off in a hurry, and using any kind of ligature strangulation (e.g. with a rope) carries a real risk of damaging the trachea or other soft tissue in the area. It's also iffy about cutting off the blood supply. The safest way to breath control it is with a hand over their nose and mouth, rather than applying any pressure to the throat or neck. You can do it to the throat and neck, but you need to be especially careful and apply the pressure over as wide an area as possible.

The most important thing is to keep in contact so you can see the state your sub is in- as flying said, a non-verbal safeword is a must, and you need to watch them like a hawk, too. Start very slowly, is the best possible advice, and don't build it up too fast; the eroticism is from the control and trust, rather than the physical sensations themselves.