Detune the strings. I know you already know this, but it is the most important thing you can do, so it is worth repeating.
Get a box from a guitar store. Pad it well in the case, and put it in the box. You want it to NOT move in the case, and you want for it to take a fairly large hit for the case to move in the box. The easiest thing is to surround it in the box with packing peanuts, though that is really messy for the guy who has to unpack it. Myself, I use a bunch of smaller boxes and bubble wrap to support the case. When you are done, you should not hear anything moving in the box.
Ship UPS or FedEx air. Do NOT ship USPS or FedEx ground. At least, not if you want the guitar to arrive in one piece. I know that a bunch of people are about to come in here and start talking about all their bad experiences with UPS, but they have a much better record than anyone, and to be frank, 99% of the problems people have had with UPS are because they packaged their stuff poorly. Package it right, and it is not a problem.
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"Cowards can never be moral."
M.K. Gandhi