When choosing a pack, a good thing to look for is minimalistic packs with only one compartment. Every zipper, every seam and side pocket, adds to the packs weight. I would rather use dry bags to separate gear within my pack as these also offer some extra protection for your gear. You can buy them in different colors and create a color-coded system, so you will know for sure what is where.
Most packs on the market are loaded with features nobody will use. If you already own a pack, it’s a good idea to cut off unneeded straps and features you’re sure you don’t need. When buying a new pack make sure to get one that has large exterior mesh pockets for storing wet gear and stuff you will want to have fast access to. Don't fall for any sort of special ventilated carrying system, which will also add weight to your pack. When active while carrying a backpack, you'll sweat on your back no matter what. The only thing that prevents excessive sweating on your back is regulating your clothing layers according to your activity level. Another thing that works for me is every once in a while tighten your hip belt and loosen your shoulder straps to air out your back. When you do this, make sure to close the sternum strap so your shoulder straps will stay in place. For comfortable backpack carrying I recommend to choose clothing which lies as flat as possible on your body. Belts are your enemy as they will get in the way of your hip belt. Choose underwear with flat elastics & seams for maximum comfort. Think of how bike couriers dress. Just wear some running shorts over your baselayer. For minimalistic backpacks, I would recommend checking out Hyperlite Mountain Gear, Gossamer Gear, Huckepacks or ÜLA. In my opinion these brands build some of the best packs money can buy.