Before you travel...make a list. Then check it twice. Think about activities you have planned. What's the weather going to be like? Will you do laundry? Can you wear something more than once? Can your clothing pull double duty?
Keep your toiletries tiny. I know TSA has their 3 ounce size requirement for carry on luggage. You can find many small bottles of necessities in most stores. If there is something special you need or small reusable bottles...check out www.minimus.biz which sells more than 2,000 travel sized items.
When you are packing that suitcase, layer carefully. In a wheeled suitcase put heavy items like shoes at the base by the wheels. Work your way up to the handle end for better balance.
Put a dry cleaning bag on each layer of clothing so items can move around and not wrinkle. Then fill in every little space with scarves, socks or small items.