The best time to negotiate your salary for a new job is the moment AFTER they say they want you, and not before. Then it becomes a game of chicken for each side to name an amount first, but it is to your advantage to get them to give a number before you do.
"Innocence always calls mutely for protection when we would be so much wiser to guard ourselves against it: innocence is like a dumb leper who has lost his bell, wandering the world, meaning no harm."