Keep it in a garment bag, give it to a stewardess to put in the flight closet. If the closet is full fold the bag carefully and place it in the overhead compartment. Make sure it doesn't get crushed up there by someone elses carry-on.
Side note: But a no-wrinkle suit. Seriously, my favorite suit is from the JCrew outlet. Cheap, fits amazing and looks amazing and never wrinkles. I kept it folded up below my seat on the airplane to my last interview with no problems whatsoever.