When you're packing boxes, pack the small items in the large boxes and the large items in the small boxes. It's a matter of physics. Heavy items are likely to come crashing through the middle of a large box. While this makes sense for most things, books are an exception. If you fill a big box with a bunch of small books, the weight is going to add up and the box is going to be too heavy to carry. Also, pack heavier items at the bottom of the box and lighter items toward the top of the box. And remember: If you're straining to pick up a box, it probably contains too many heavy items, and it could wind up breaking.