What is the best way to package a guitar to ensure if will arrive in its destination without any harm? Lots of bubble wrap? A box within a box? Loosen the strings? Should acoustic and electric guitars be packed the same way?
Loosening the strings is a must if your guitar is likely to fly. I took a flight to Germany in 2001 without loosening the strings of a Strat I had at that time, the guitar was in a hard-case; when I got there the springs had somehow slipped of the back of the tremolo causing the tension of the strings to pull the tremolo up to an obscene angle. I put this down to the difference in pressure in the cargo hold of the plane (the plane I flew in that day was very old; short haul Lufthansa flight from Amsterdam; it had propeller engines); though I'm still not entirely sure why. The guitar turned out to be fine, but ever since then I have either tried to get them on as hand luggage; or semi-detuned the strings. Whether you need to do this with a modern plane I'm not sure, but perhaps better safe than sorry.
Also a hard case would be advantageous; then put the guitar contained within the hard case in a box padded with this kind of stuff (this is how a manufacturer might send you one). Make sure the box is sturdy and made of pretty thick cardboard.
If you don't have a hard case; then bubble wrap round the guitar/or a soft padded gig-bag, then put that within a box; then put the boxed guitar in a larger box, padded as described above.
I think acoustics should be no different; again a hard case is preferable.
For maximum safety, do this:
Here's a link.
I would place bubble wrap around the guitar and then place it in a hard case. In all the empty spaces that could cause the guitar to move around I would stuff with a soft material (foam/bubble wrap), and that will stop the guitar moving around in the case during transit.
Pay attention to the neck, wrap it in bubble wrap fully. If the guitar has a Strat-style 'whammy' bar, I would unscrew and remove it.
Loosen the strings, and throw in a new pair of strings as a good courtesy, in case a string breaks in transit. If you are selling via eBay then this may earn you more positive feedback (it shows your thought and courtesy)
If the guitar is flying in any way, then the hard case is the most important thing. No matter how much 'Fragile' tape is put on it, we all know what can happen at baggage handling.
Hope this helps.