All hiking backpacks are not the same; they are designed for different uses. It is important to give some thought to how, and where, you are going to use your new hiking backpack. After all, you will have to carry it up and down hills, jumping streams, scrambling over rocks, and still have enough energy to enjoy your hike!
Your full pack should not weigh more than about 25% to 30% of your body weight. Carrying a lighter load will make the hike easier, and also help prevent accidents caused by fatigue and unbalanced backpacks.
The size of your hiking backpack depends on what type of hiking you usually do. If you only go on day hikes, with an occasional overnight, then you should opt for a day backpack. If you mainly go on week long trips, you should get one of the mid to larger backpacks.
The larger hiking backpacks have padded shoulder straps, and I would recommend them also for day packs. You may not be carrying as much weight, but there is no reason to be uncomfortable, and sore shoulders can ruin even a day hike.
Waist belts are a must for larger hiking backpacks. Not only do they transfer most of the weight to your hips, but also ensure a more stable and balanced load, which prevents accidents when climbing over rocks and moving over rough terrain. Most backpacks also have smaller straps, which fasten across your chest, for even more stability.
Another feature of most hiking backpacks is external pockets. Use them to store items you want quick access to, like water bottles, medical kits and other small items. Large zippers in the lower part of a backpack make it easy to reach stuff packed at the bottom.
You do not need a clean set of clothing for every day. Get quick drying hiking shirts, and wear one while the other dries. One pair of boots is enough, but pack about 4 pairs of socks. Carrying enough drinking water, and food, is much more important than an extra pair of pants.
Size your new hiking backpack to accommodate everything you need to take with you, without stuffing it to capacity. An overloaded bag will usually be very uncomfortable, and can be dangerously unbalanced. Rather select one a bit larger, and resist the temptation to fill it just because there may be some space left.