The general principle is to pack the rucksack in such a way that you can get to everything where you need it easily.  For example, if your stopping for lunch, you dont want your food at the bottom of your rucksack under your sleeping bag.

With a bit of thought and practice you become experts on your own rucksack.  Then when you buy a new one, you have to work it all out again and with a bit of experience you will make the best use of its design.

A few hints:

• Don,t go cheap - along with footwear and waterproof, buy what you can afford.
• Make sure the rucksack fits you
• Ones with lots of zips, straps and pockets are complex and there's more things to break/snap.  Simple and sturdy is better.
• Practice putting it on and off safely.  Put it down with care - dont just drop it.
• Make sure its got a rain cover and use it.
• If your going to attach items to the outside then do it in such away that it is secure and streamline.  No swinging water bottles or cups.
• If your going to sit on it then be mindful of whats in it.
• Double bag liquid fuels and rubbish.
• Belt buckles are fragile and indispensable so look after them.  Many a buckle has been trodden on and broken.

The image below is just an example and not a definitive way.  It show the general idea.  Practice, experiment and refine.