Step ladder, A-frame letter, cherry picker. Best not to go on the roof and hang over working below, that's never a good thing.
Other than that, maybe use ropes to anchor yourself to the roof; i.e. a counter weight that's heavier than you (at least two times heavier) on the opposite side of the roof either on the ground or the roof
Is your house a single story, or is it two? How steep of an angle is the roof?
I never recommend working below your self while on a roof, it's just plain dangerous. The safest was would be to stay on the ladder.
That being said, in the likely event that you ignore my suggestion, go to your local fabric store and get two nice big (2 feet x 2 feet or so) pieces of 2 inch thick upholstery foam. Keep one of the foam pads between you and the roof at all times; it will keep you from losing your footing and sliding off the roof.