1. Know your needs. Packs are essentially divided into day packs and full sized backpacks for multiple days in the backcountry. Day packs are used when you need to carry the minimum and you are out for few hours to enjoy the outdoors. Backpacks are used when venturing for several days and you need to carry your own camping gear, nutrition and emergency kit.
2. Before buying a full sized pack decide what capacity is right for you. Day packs usually have volumes between 30L and 40L and can vary between 9kg and 14kg. It is always important to choose one with a rigid frame and padded waist and chest straps. Keep in mind day packs are also used by climbers to carry climbing gear, helmet, ice axe etc there fore some day packs have extra features like ice axe loop, crampon, belay devices, rope etc.
3. Overnight or backpacking requires 50L to 80L that can carry about 13kg to 25kg depending on the demand of the climb. Longer trips and expeditions combs require more. Padding is very important when choosing a backpack. It will give you comfort when carrying the extra weight. Choosing a backpack with adjustable top is recommended as it gives you extra space. That is why you see most backpacks rating state the Litre amount + 10L.
4. Loading a any pack is very important as it will influence any climbers speed, endurance, and enjoyment of the outing. Best option is to concentrate the load on the hip/waist to avoid having the load on your back and shoulders. Pack heavily items as close to your back as possible and well centred to lower centre of gravity for better balance while climbing. Lighter items can go on top of the pack. Adjusting and readjusting the backpack during use helps reduce soreness and fatigue.
5. Managing pack chambers/ pockets is very important. The gear that will be needed the most is often carried close to the hand. For example gloves, hats, sunglasses are carried in side pockets or top pockets for easy and fast access.