During my cross country move, I found that you need to allow at least 2 twice the amount of time to pack that you originally had planned.  Getting distracted by the stuff you own is easy and time consuming.  Also, while looking at the stuff you uncover while packing as yourself 3 questions:

1. Do I need this?  2.  When was the last time I used it?   3. Can it easily be replaced?

The need for an item is not just useful but sentimental.  Do you need it to be happy? is a very important part of that question.  If it is an item that you think you need but really never use, maybe you should rethink the use.  Last, if the item cannot be easily replaced then you must take care to pack it carefully and with thought.  If it is fragile, you'll need bubble wrap and a good sturdy box.  If you are using cardboard, you may want to think about re-enfored cardboard (two-ply) because a cardboard box can easily get crushed under the weight of other boxes and items.  If you use a plastic box, be sure the lid will not collapse inward with weight on it. 

If you have time a 3x5 card with a list of the items inside is a time saver when looking for something.  Also, do not be afraid to group odd things together if it fills up a box.  You will want the most amount of moving for the least amount of movement.  In other words, filling a box to the brim, means less boxes to move.

Never allow anyone else pack something you'll blame them for breaking. 

Good Luck.