Purchase luggage cubes. These fabric, rectangular casings often have mesh covers and are zippered. You can fold or roll clothing, and stack them in the cubes. The cubes fit easily in luggage and save you space.
Roll your clothing. To do this, fold clothing vertically in half first. Imagine folding a pair of pants so that one leg matches the other or folding a t-shirt in half the same way. Then, roll the clothing tightly from the bottom of the vertical end to the top. Place in a luggage cube or in your carry-on bag.
Pack one pair of underwear, one bra and one pair of socks for every day that you will be gone. If you plan to be near a washer, then pack half this amount. You can wash your items and then let them air dry.
Consider the weather where you will be. By not having to pack bulky items --- such as jeans, jackets, bathrobes and even tennis shoes --- you can save room in your carry-on bag. Can you get away with wearing one pair of sandals on your trip? Can you wear shorts the entire time? How about sundresses? These items pack well because they can be folded tightly.