Supplies: Sturdy boxes, packing tape, bubble wrap or other packing material, marker, notebook 

• Consider donating or selling items that you never use. 
• If you will know you will probably never use something, but can't get rid of it for whatever reason (wedding gift), consider boxing it up and never unpacking it when you arrive at your destination -- that is what basements and attics are for.
• Use sturdy boxes. 
• Use the sturdiest of those sturdy boxes for the heaviest items.
• Boxes of the same size are easier to stack. 
• Consider asking for used boxes at a business that uses a lot of copier paper.
• Place bubble wrap or some other cushioning material around fragile items.  An efficient (and free) packing material is the towels, bed sheets, and even clothing that you have to pack up anyway.  
• Fully fill the boxes to prevent them from collapsing if other boxes are stacked on top of them.
• Label each and every box.  A good way to do this is to number the boxes and then write in a notebook next to the corresponding number a detailed list of the contents.
• Write "fragile" on any boxes that contain breakable items.
• Don't make the boxes too heavy.  If a box of books is beginning to get heavy, consider filling the rest of the space with stuffed animals or something else light.
• Use lots of tape.   Forgoing 3¢ worth of packing tape won't look so bright when you are on your hands and knees scavenging for the 10,000 microscopic Polly Pocket toys that spilled out onto the driveway.
• Stack boxes so that the sturdiest, heaviest boxes are on the bottom and the lightest and most fragile are on the top.
• Don't stack the boxes too high -- the bottom box may not survive the weight or the stack may become unstable and topple over.