the clothing, lighting, music, models.
all of the above has to be seamless, and a blend of perfection for a fashion show to be a success.
the clothing is the star.
trhe atmosphere and models, help to 'sell it'.
In addition to the excellent points made by Marryanne, put some thought into the off stage elements. For a successful show one needs perfection on stage and off. Keep in mind proper selection of stylists, back stage co-ordinators, etc. Make sure to have good fashion photographers and videographers to capture the event. Photos and videos are needed. Keep in mind that you want more people to see the show then the people in the room (or tent). Video and photos are needed to spread the word...
Obviously, let's not forget that the crowd is important. Celebrities, buyers, journalists... Hey, maybe I should be invited to all successful shows (kidding).