Bubble wrap, freezer bags, and garbage bags!
Some people are big "shoppers" whenever they attend conferences (or travel in general). If you plan on buying an item that might break, pack some bubble wrap in your luggage. Folded flat, it doesn't take up much room, weighs nothing, and could save your souvenirs. (I did this when I went to Italy last year. Got my bottle of Lemoncello back to the states in one piece!) I also like to pack items in zip freezer bags. To keep things organized, I put scarves in one, socks in another, etc. If you press the air out of them and compress them as flat as possible, they take up less space in your luggage. They are also lifesavers (clothes-savers?) in case your lotion, shampoo, etc LEAKS. A garbage bag or two is also useful for dirty clothes (if you're not doing laundry while you're away).