Based on my experience, remove the turret if it requires no tools (e.g. Tiger1, King Tiger, Panther, Sherman, Pershing, etc.). Bubble wrap the upper and the turret separately, then tape them together. Put the assembly in a plastic garbage bag (that will catch any broken bits if they come off during transit). Pack securely to keep it from shifting during shipment (packing peanuts work good).

If it's a tank that the turret doesn't come off easily on, then I recommend wrapping in bubble wrap, and then taping the assembly to a larger piece of cardboard custom fit for the box bottom. The cardboard piece should fit tightly. That should keep it from sliding around. Packing peanuts or stuffing as per usual to keep it from shifting.

Make sure to bag everything, and tape it all together, that way nothing gets lost.