Most people know that rolling T-shirts is the most efficient way to pack them into a suitcase, but not all shirt rolls are created equal. For a truly tight suitcase, you should master the military-style roll.
Jolie Lopez shares this trick on Snapguide. It'll be easier to follow along with her slideshow, but basically you need to start by folding the bottom of the shirt up to get two flaps on the outside of the finished roll. You can pull one of these flaps back over the rest of the shirt to make an incredibly tight t-shirt burrito. This tip is pretty specific to T-shirts, but it's definitely worth learning next time you go on holiday.