If you are stacking your boxes, make sure there are no open spaces inside of them. If you have drawers, garbage cans or crates - don't let that space go to waste. Fill them with other items.
Storage areas which will be accessed frequently should be packed differently from those which will be unpacked all at once. If you need to access your belongings on a regular basis, you should put spaces between the boxes to function as aisles.
Remember that more space costs more money. Anything you can do to keep your belongings compact is a good idea.
If you are storing items which contain liquids - such as humidifiers, chain saws, lawn mowers - be sure to drain them of their liquid.
Cover cloth furniture (sofas, mattresses) with plastic. This can help protect them from dust or humidity effects.