I find the middle finger just to the front of the earhole and index finger on the bony bit behind the ear help best. That way I can block out the nasal soprano behind me who knows 2 volumes, loud and louder, and send her a rude hand gesture at the same time.

Mind you... I've got an oddly shaped ear canal so it might not work for everyone. Plays merry hell with that machine with the torch on it that doctors use to look into ears (auralscope?)

LTS