Roll it and protect it
Before you actually pack the bag, you should take precautions to ensure your toiletries won't leak everywhere, and that your clothes will come out wrinkle-free.
Put anything leakable into plastic baggies or in waterproof carrier bags. This includes any and all makeup, shampoos, shaving cream and moisturizers. Set this items apart from other items, as you'll want to carry these on board with you.
Pack shoes in an old pair of socks to prevent shoe polish from rubbing off on clothing or your suitcase.
Roll 'em up!
Linen aside, most fabrics can escape from a luggage bag without wrinkles. It's all in how they were placed in the bag.
First: Make sure the clothes aren't wrinkled to begin with. It doesn't matter if you're an expert at rolling--if the clothes aren't smooth at the start, they won't miraculously uncrease themselves in a garment bag.
For shirts and dress shirts, fold the arms in, into the back of the shirt, at the crease. Take the bottom of the shirt and tightly roll up the entire shirt, making sure the arms are tight against the back and are rolled as well. Leave the collar at the top and place in the bag. White dress shirts should not go on the bottom of your bag, where they could be crushed.
Skirts are very simple to roll up, just lay the skirt flat out on a bed or table, smooth the fabric with your hands and roll up.
Fold jeans and pants at the crotch, lengthwise. Make sure both legs are symmetrical, and carefully roll them up together, cuffs first.
Rolled clothes should stay in place if placed tightly against more clothing or other items in your bag. Now you're ready to start loading up that bag!