My seamstress gave me a good tip:
put a sheet, preferable soft cotton so the dress can breath, over the dress, then put a plastic garment bag over it, then another plastic garment bag, then a harder garment bag over it.
Ask the stewardess, they have a cabin to hang your wedding dress in the front of the plain.
Bring 100% soft Cotton (baby cotton) sheet (same sheet you use to cover your dress or a pillow case) - you can use it to smooth out any crinkles or wrinkles in your dress by lightly going over it with the cotton sheet.
Also, you can buy a hand held steamer to use - MAKE SURE YOU TEST IT OUT FIRST on a piece of fabric (same fabric as your dress - the boutique or seamstress you bought it at should be able to give you a swacth).