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.
A wise man said: before you speak, let your words pass through three gates. At the first gate, ask yourself, "Is it true?" At the second gate, ask "Is it necessary?" At the third gate, ask, "Is it kind?"