I found garment bags wrinkle you clothes the worst. If you plan on using it, make sure it is full. The less in it, the more things get tossed around and crushed by other suit cases. If it's packed rather tight, the clothes will keep their shape a little better. Great if you want to share a bag with your cabin mate. But I can't fill one up myself.
I find everything layed flat, stacked, then rolled and put in a back pack provides the least wrinkles. It depends on the amount of clothes and size of bag you need.
If taking a suitcase, Pamda's method usually works the best; everything flat, stack them in the bottom of the suitcase and fold up sleeves, pant legs and anything else hanging out.