So much like the boots first few paragraphs, do some investigation, what are you going to do with this pack? is it a 'day-pack' a 'multi-day pack' or a 'week plus pack' which could be a 45L, a 60L or a 95L. You need to identify the packs use. Let's just say you want a pack for day trips only. If your hiking, it might need to be 30-45L pack to hold your gear needed for that days adventure.
Let's say you're planning a couple of days into a hut, you might need 50-75L pack. Let's say you're planning a huge across New Zealand adventure which will take 12 days, perhaps that 75-95L pack might be the one needed. Remember, the bigger the pack the more you will take and you will probably take stuff you don't need to take or won't even use.
Choosing the right size pack for the gear or equipment you are going to carry.
Again don't buy the first thing you are shown, test lots out, ask friends, other experienced hikers or hunters about their pack and what they use and why they use that type of pack. They use that pack because it works well, is comfy, holds all they are carrying with extra room if needed, has never broken down or failed, it's ease of use etc etc.
Experienced hikers or hunters need their packs to do their job and there is a reason they chose the ones they are using.
When you decide on the pack your going to buy and it's for the purpose needed, as I've mentioned elsewhere, look after it. Keep it clean, if it gets dirty, wash it and make sure it is dry before putting it away.
Look after your pack, when it gets dirty, clean it and it will last alot longer.
If you buy a good quality pack and let's say you spend $400 on one,(you could spend $1000 on one) it should last for many years. Also look at buying a good quality pack liner that all you're gear can go in inside your pack. This keeps all you're gear clean and dry, if you are out in the hills and it rains, you're pack will get wet but all you're gear will stay dry. A good investment.
I've also mentioned to look at the 'less is best' a simple tube type pack, opens at the top by a draw string and then the top main cover with compression straps locks in place over the opening. There is just less that can fail with a simple tube type pack. A good lightweight pack and frame with good harness support system is important as well as a good secure hip belt. All things you need to look at when buying a backpack. Pack weight, it's worth looking at, my latest pack a Salewa Apex 45L Guide is 825 grams as opposed to my older MacPac at 2kgs. So a good reduction in weight by just looking at pack weights.
The 45L Salewa Apex Guide, 825 grams.A very lightweight pack.
Also remember there are pack designed for females that are a lot more comfortable for the ladies so keep that in mind. New modern packs can be adjusted to fit your size and shape so test them out.
There are lots of fancy packs 'Out There' with all manner of zips, clips, domes, straps etc, don't be fooled into thinking 'Wow this is awesome'. Generally speaking the old saying 'Keep it simple' is the best option. And look at buying bright coloured packs, red, yellow, orange, because if you put your pack down in the bush or tussock and walk away ( for whatever reason) they are easier to find than a fancy camouflage one.
My 55L MacPac Pursuit, used for a 4 day hike, up the Greenstone and down the Caples valleys.
As a rule you should never put your pack down and walk off as everything you need to survive is in that pack and if you are separated from it you might have a pretty horrid night ahead. But it's alot easier finding a bright red, orange or yellow one than a camouflage one. One good tip is to get some reflective tape or a square of reflective card and attached to the top of your pack. Then if you did put your pack down and lost it, in the dark when a light flashes onto the reflective tape or card you will be able to pick it up.
The 45L MacPac Pursuit an idea pack for those multi-day hikes.
One thing also that I find is good on a pack is having a side pouch, could be either side of the pack, could be on the front and this is where I store my water hydration system, a platypus, a 2L water pouch that has a tube you can drink from so you can drink on the move. Some people might just carry a water bottle. I use the platypus system. Anyway, stored on the outside of your pack, if it should leak it won't leak the fluid into your pack where all your gear is. As opposed to it being stored inside your main pack with all your gear. However if all your gear in stored in a waterproof pack-liner it shouldn't matter.
Just like boots, a good quality backpack, if looked after will last for years. I've got one pack I'm still using and I've had it for probably over 30yrs. Buy good quality and again you won't regret it.