Whether you are carrying an internal or external frame pack, you should load it with balance and the convenient location of gear in mind.  a few basic packing principles apply to both styles of packs.

• Remember to keep in mind that whatever goes into the pack first will come out last.
• Protect you sleeping bag.  Internal  frame packs have a special sleeping bag compartment in the bottom.  External frame packs have a special area below the pack bag where you can lash on your sleeping bag.  In either case, make sure your sleeping bag is well protected from the elements.  It is always a good idea to carry a large plastic garbage bag.  If it rains, you can line your sleeping bag stuff sack with it for extra protection.
• Balance you load.  Carry clothing, cooking essentials and food in the main compartment.  The heaviest gear should be stowed toward your back and centered in the pack to provide proper balance.  It helps to use a soft garment as a buffer between your back and hard-edged items such as cook kits and fuel bottles.  Mid-weight gear should be carried toward the top and outside portion of the pack.  Organizing your gear in color-coded stuff sacks makes packing easier and helps you locate supplies quickly on the trail.
• Keep essentials handy.  Decide what you'll need during the day.  What does the weather look like?  Will you need easy access to rain ponchos and plastic bags to cover the packs.  Or will you want to change into shorts a midday?  A change of clothes, along with sunglasses, suntan lotion, maps, candy bars, and other small essentials need to be within easy reach.  Lunch and munch food should also be easily accessible.  Plan to take pictures at every scenic point?  If so, keep your camera in a handy pouch.
• If you will be hiking or getting into camp in the dark, make sure to pack your flashlight or headlamp where it's quick and easy to get to.
• The easiest way to avoid frustration is to consistently pack the same items in the same pockets.  Once you have a packing routine, you'll find it natural to reach for the right pocket every time.
• Aim for no more than one-fourth of your body weight to start, one-fifth of your body weight if you're out of shape and already hauling around a few extra pounds of your own.  Very fit hikers can handle one-third of their body weight, and in some cases more.
• If there's any chance of wet weather make sure to pack in waterproof bags.  Garbage bags will work.  Gallon sized zip-type bags work great for sets of clothes.  Also remember to make sure that if you have to open your pack in the rain that you haven't put everything in one garbage bag.  If you put like items in smaller bags, you will be able to find things you need without everything getting wet.
• Many times when backpacking you will be sharing gear with another person.  Even if you intend to walk together at the same pace, make sure each of you has complete useful items; The person carrying the stove should have the pots and the fuel; the person with the tent gets the whole thing, including poles.  The person with the stove should carry the ground cloth or space blanket; the person with the tent gets food that can be eaten cold.  That way, if you get separated by accident, you each have gear you can make use of.
• Some liquid gas-fueled stoves come in two parts; make sure you've got both parts of your stove.  Also if you stove has a specific fuel bottle as part of the assembly, make sure that the bottle you take fits the stove you're going to use.
• Make sure you have your tent poles:  The right number of the right kind.  Ditto for tent stakes.
• Another item that seems pretty straightforward is to make sure that the waistband buckles are both there.  I once reached the trail-head and tried to snap the plastic buckles and realized that one of them was missing.  A carabiner worked in a pinch but it sure wasn't comfortable.
• Keep your gear straight.  Stuff sacks are backpacking's answer to file cabinets.  Go for bright colors:  you won't end up leaving one behind by accident. (Don't worry - you're not violating low-impact courtesy - your stuff sacks spend most of their time hidden your pack, right?)
• Once your pack is packed check out the fit when it's loaded down with actual gear.  Make sure that your shirt doesn't have seams exactly under you pack's shoulder straps, and everything that's supposed to fit inside your pack actually does.
• Last but not least, if you're the person with the keys to the car, make sure that they are easily accessible when you get to the car.  You don't want to have to dig through everything to find them once you get to the vehicle.