Originally Posted by a_elephant

Just to add to the info already here:

A good, sturdy keyring is also really important in keeping your keys secure. I learned this the hard way when I lost a ring of keys NOT due to carabiner failure, but keyring failure while crossing a bridge on foot in Dominica. The keys were on a very flimsy little metal ring, about the strength of a twist tie like you'd use for a bag of bread (given to me this way, not hand-selected by yours truly). The ring stretched/bent and the keys came flying off into the water below. The harder it is to put your keys ON the keyring, the harder it will be for them to come off. I always use a very stiff keyring now, and I'm on my way now to REI to buy a locking carabiner.