The few more or less obvious points (bought 4 synths across europe the last couple of years):
- Put foam or wrinkled up newspaper under the keys, at the front of the keyboard to prevent the keys from moving/breaking.
- Seal it from water/moist with a big plastic bag/sheets close around the keyboard and tape it shut.
- Then put bubblewrap around it (the one with bigger bubbles seems good).
- One seller used empty plastic water bottles withthe screw cork on as air cusions around the synth, seemed to work pretty well.
- If using foam packing "peanuts" make sure the box is sealed well so they stay in the box.
- Pay attention to the "ends" of the package as people will rest/stand the package on end.
- If you don't feel confident dropping the package from like a foot or two - pack more
When receiving a keyboard I always do the "rattle test" (lift it and tilt it and turn it upside down) before even thinking of plugging anything in, if something is loose inside you need to check what it is - I've had screws and even a metal paper clip loose which could easily have come into the power supply.
Check any voltage selector (110 - 220V) is set right.