[sloaken]

Rule one: wait until you have your next job.

Why are you leaving? If you are going to go to school, then you might want to come back later, I would start dropping hints.

Do you need a sabbatical? Some places can deal with that.

Since they do not want to loss you, by HINTING, not telling you might be able to work something out.

If you are retiring, then I would tell them now. They will appreciate being able to bring on a new person and having you train them.

If you are going to a competitor, expect to be bum rushed out the door. Do you want to be able to come back? Or do you despise these people? In general it is never good to burn bridges.