Have you ever used those space saving bags inside your luggage when you travel? They shrink the clothes in them by removing the air. If you are taking bulky sweaters, jackets, or any items that take up extra room...these work great.
You can buy the travel version of these that don't need a vacuum to operate. Or, you can try it on your own with large zip topped bags. Fill the bag and squeeze out as much air as possible. You will be amazed at how much space you can save.