There's a great packing method wherein you create a bundle of clothing within your suitcase. Socks, underwear and pajamas make up the bulky center and wrinkle-prone garments such as slacks, shirts and jackets make up the outside of the bundle.
You basically place all of the articles in your suitcase, starting with the most wrinkle-prone and ending with the center items and then wrap layer by layer around the bulky center.
I tried it on my last trip and was absolutely amazed at how well it worked. Do a web search for packing methods to read a better description of this method, but I highly recommend it.