A couple of general things, first:

Are you male or female? I've read that, for a female, heavy items packed lower may work better. (Since I'm a male, all I can do is pass that nugget along; I'm not about to undergo radical surgical procedures just to see if it's true. <img src="/forums/images/graemlins/blush.gif" alt="" />)

What is the total weight of the pack you carry? Back when I carried 40 or 50 pounds for a weekend (definitely NOT "the good old days"), using an external frame pack, heavier on top was definitely more comfortable and easier to control. Now (and "THESE are the good old days") when I'm carrying about 16 or 17 pounds for that same weekend, I find it doesn't really matter where anything goes - there's just not enough "anything" to create problems.

Internal or external frame? I found that the external frame pack was more sensitive to loading than the internal. My internals seemed to work best when heavy items were above the center, but not completely in the top (say, the middle third of the pack) - but it also seemed to make a big difference whether they were close to my back or close to the outer edge; close to my back and in the middle third for the heaviest stuff, then getting lighter as you moved up, down, and out.) Again, nowadays, it doesn't seem to matter.

There's another general principle: pack the stuff you're most likely to need, and the stuff you need most often, during the day in outside pockets or on the top of the pack; it's more convenient.

When I'm using my Vapor Trail internal-frame pack, I tend to put my sleeping bag in the bottom, tent on top of that, then food and stove. Spare clothing, if any, tent poles, empty water bladder, and toilet paper goes along the outer edge of the pack; raingear goes on top of the food and stove, and the self-inflating sleeping pad (rolled, or folded into a flat, square shape) goes on top of the raingear. Water bottle, filter, and map case go in the outside pockets.

The only change I make when I use my Virga frameless pack is that the sleeping pad gets folded into fourths (once lengthwise, once widthwise) and inserted down the back of the pack, where the sleeping bag locks it into place as my "virtual" frame; I sometimes give it a couple of puffs of air to stiffen it. Tent poles sometimes move toward the center of the pack, along the pad, to act as a stay and further stiffen the pad if needed.

Mostly, just experiment. You'll find a method that blends the most comfortable carry with an acceptable degree of convenience. Hope this helps.