I've also found using cardboard boxes within suitcases helps, as does buying a suitcase with some kind of frame inside it (like the rectangle rolling ones). Bubblewrap, as laragoth says, is a must. I also like socks. They're really great at wrapping around things. If my suitcase has a liner I can get into, I'll slip cardboard panels into there (I just use cereal boxes cut up), which gives it enough structure it can bounce off some things that would normally smoosh my suitcase.

Another easy way to eliminate any issues: when you are speaking to the service clerk at the desk to check in bags, etc, just say "In the bag I'm checking in, I have something that looks like a weapon/is a weapon". They allow you to check almost anything (weapons are allowed, with some restrictions), and they like it if you don't surprise them. For toys, all they ask is you stand by the scanning area while they scan your bags, and so you are available to pull things out of your bag and explain what it is if the scanners have questions. They also had us do this for my husband's bastard sword (real) and my bow (also real). The only time we've even given them pause is one of our steampunk guns looks like a real kiddy shotgun (I guess we did too good of a job painting it) and they had us pop it open to make sure we didn't need to sign any forms. So, just show up a bit early to allow them to inspect your bag and allow for repacking time.