The advantages of having a lightweight, frameless backpack are huge - but without that heavy frame, the backpack has all the qualities of a large sack. For maximum comfort, you'll need to pack carefully, in a way that gives your backpack form and balance.
On a partial hike of the Appalachian Trail, I learned the importance of packing correctly using a GoLite Gust, a durable and capacious pack that weighs a little over a pound. My sleeping pad became a frame, and my tent poles gave the pack structure. It took me a while, but eventually I figured out a system that worked for the Gust. Depending on the materials available (i.e. your gear) and your type of pack, the exact formula will vary. However, you'll need to consider form, balance, and functionality when packing your ultralight backpack, regardless of its type.
Ultralight packs come in a wide array of sizes and styles - if they don't include a frame, you'll need to build one using your gear.
Before you do anything, make sure your pack is waterproof. Unless you want to carry a bulky raincover, drop a lawn-and-leaf or trash compactor bag into your pack. This provides a waterproof buffer between the elements and your belongings. Since the material of most ultralight bags is quite thin, your pack won't retain much water. It'll get wet, but it won't get heavy, and that's what matters. Spraying your pack with silicone waterproofing spray is another option, although the coating will wear off over time. Pack your gear into Ziploc bags for another layer of protection.
To build a frame into your pack using your gear, the two most important items are soft padding to go against your back and a stiff frame to give it form. Internal frame packs have these features built into them, but that becomes redundant once you realize that the items you commonly carry with you can be repurposed to create a frame.
If you've ever taken a pack apart, you may have noticed a thick foam pad that looks a lot like your sleeping pad. Why carry two near-identical objects on your back when one can serve the purpose of the other? It's this that I love about the ultralight mentality - eliminating redundancy. By folding your sleeping pad in three, you can create padding for your back. With the GoLite Gust, curl the folded padding so that it fits more easily and stuff it into your pack so that it forms a nice solid foam back for your pack.
If you're carrying tent poles, they're great for giving form to your pack. Arrange them so that they run parallel to your spine when the pack is worn, and stuff your other belongings around them to keep them upright and the pack rigid.
Tent poles plus folded sleeping pad equals a makeshift frame for an ultralight pack.
Although weight distribution matters less for ultralight packs (there's less weight to go around), it can still make a difference. For optimal comfort, keep your heavier items close to your back and near the middle or top of your pack, depending on what you find comfortable. First, drop your sleeping bag into your bag so that it ends up on the bottom of your pack. You don't want too much weight in the very bottom of your pack; a lightweight sleeping bag is ideal.
If you're like me, food is one of your heaviest items (I once ran into the “GoLite Guy,” Demetri Coupounas, hiking the Long Trail with a backpack full of kelp - yuck!). Split your food into two bags and sink one down each side of your pack to balance the load.
To distribute the weight of items like stove, fuel, and headlamp, wrap each item in a piece of clothing. This protects your gear and silences items that might otherwise bump together and clank as you walk down the trail. Be consistent so that you can find your gear when needed - for example, my headlamp always lives inside my woolen hat, near the middle of my pack.
Retain a small bag filled with your food for the day and keep that near the top of your pack so that you don't have to dig down into your pack until the end of the day when you make camp. Pack other items you might need during the day, such as your rain jacket, near the top of your gear. If it begins raining, you won't have to risk getting all your other items wet while digging out your rain gear. Pack your rainfly or tarp above your sleeping bag. When setting up camp, if you can get the rainfly up first, you can set up your camp in style without getting any wetter.
Whether you're bringing water bottles (I prefer Gatorade) or a hydration bladder, keep it accessible. Pack it near the top of your pack or in the front pouch.
For the sake of consistency, once you've worked out a system that works for your pack, stick to it. This will make packing your bag a habit instead of a chore, and you'll know where each object lives in case you need to get to it quickly.
Remember, don't overload the pack, even if it's as accommodating as the Gust. The thin shoulder straps of ultralight packs aren't designed for heavy loads. To keep your pack comfortable, stick within the manufacturer's weight recommendations. The Gust gets uncomfortable around 30 pounds, and does better with loads under 25 pounds or so.
Hiking part of the Appalachian Trail with my Gust pack - my first extended backpacking trip - I came across a fellow hiker with the same pack. He had socks stuffed underneath his shoulder pads and, loaded down as I was, I thought “Great idea!” Now I know better - lighten your load if your shoulders are getting sore. An ultralight pack works only if you pack it with ultralight gear!