I would recommend against using a bag. Bag means soft and the airline will not give you anything back if they bash your bike inside of a soft bag. Use a box of some sort. I use an ABS plastic box between Europe and the U.S., with no ill effects, other than the additional cost of shipping. Different airlines have different cost structures on same, so make sure to check. Using the spacers in the dropouts, even in a hardcase, is highly recommended. The foam pipe insulation is also highly recommended, as is insuring that you tie/tape everything down well within the box. The cardboard box someone above recommended is good. Can usually get your LBS to supply one for free and pack your bike properly in it for $20, or so. Removing the RD from the hanger is very important for the reason stated. Just make sure it well secured so it doesn't bang around against the bike/wheels. Make sure everything is is well secured. If the airline can shake it loose and bang it around, they will. On one of my trips back from Europe my bike got here several days late. It came home via the Dominican Republic. Don't give the airline an opportunity to screw up. For they surely will.

You might see if you can find an LBS where you are going to ship your bikes to ahead of time. That way they are there and ready to go when you get there. Then give back to them to package for the return trip. That has worked well for me on two trips over.

Good luck and have fun! You have chosen a great area to ride/visit.