Sabien: Well, I'm afraid I've never attempted anything like this before so I have no idea if it would work, but I guess what I'd do in that situation is make the horn like a plush horn, then use a wire attached to the tip and the other side to keep it from falling down from the weight. But that's just an idea, I have no idea if it would work or not, sorry I can't be of more help.
At any rate, I tried my best to sketch the plan for my Lugia costume. I know I'm a lousy artist, but this is the best I could do. I don't even have any colored pencils with me, so I quickly added color using my computer's lousy photo-editing program. So yes, I know the picture is lousy, but it should give you the general idea.
Basically, it's all-white for the most part, except for: the spikey-eye things coming off the side of the hood, the chest part, the plate-like things on the back, and the two spikes on the tail. (Yeesh, I'm a writer and this is the best I can do to describe them.) For the tail, I'll wrap the cloth belt around my waist, and I'll line the back of the eye and back things with a thin layer of cardboard so they'll stand up. Still, I feel it's missing something.
I had to scrap my original plans for a Dread Pirate Roberts-esque Lugia, since the mask just wasn't working, as was pointed out here, so I'm going with the hood, like recommended. As you can tell, I'm not very confident in my artistic ability, so any advice or tips would be appreciated. I've never done gijinka anything before, so this is a new experience for me. If anyone could give me their thoughts and recommendations, I'd be very thankful; I'm planning on getting the remaining material tomorrow, or at least that's the plan.