So is there any really safe way to lower a hammer on a loaded gun, or does letting the hammer slip mean your going to fire it?
Decent explanations above -- I just want to comment that there's more than one way to do it, and the description is best accompanied with pictures. I'll try to gin something up in the next few days, if someone else doesn't beat me to it.
Practice with an unloaded gun, many times, until you are sure you know which technique works best for you and can do it without bumbles. And it should go without saying (but being a nag, I'll say it anyway), keep the gun pointed in a safe direction while decocking, no matter how well practiced you are.
pax