Buying: One aspect of pack purchase that's sometimes overlooked is the actual buying process. It's crucial though and worth taking some time over to get a comfortable fit - remember, everyone varies and what works well for someone else, may not be as effective for you.
If you can, go shopping at a quiet time when shop staff aren't rushed off their feet. Get them to load up the pack with a heavyish load - some good shops have special bags for this, or climbing ropes are a good substitute - and ask them to help you adjust the sack precisely to fit you.
Walk around for a bit and see just how it really feels. Does the load feel stable if you swing around? Is the weight transferred effectively onto your hips or does it feel high up and unbalanced? Are the straps and hip-belt comfortable or do they cut in and hurt?
Bear in mind that the foam used in pack padding will bed in slightly with use, but if the fit is wrong to start with, look elsewhere. It's worth getting it right, as an uncomfortable pack will ruin your trip.
At first glance, there seems to be a huge number of different back systems out there, but the majority work in roughly the same way. An internal frame, often two metal stays, feeds the load into the hip-belt area while some sort of foam padding cushions the back and allegedly provides ventilation and wicking capabilities. Simple really...
Adjustable back systems allow the manufacturers to make one size of pack to suit a number of back lengths, which is great if you intend to share the pack with a number of friends of disparate proportions, but for most people, it's a question of adjusting the pack once to fit their back length, then leaving it alone. For that reason, ease of adjustment doesn't really count. More important is security and robustness once adjusted.
Fixed back length systems mean that the manufacturer has to produce packs in different back lengths so you must choose the right length for optimum performance. They're simpler, potentially lighter and can be more robust.
All the bells and whistles in the world mean nothing if the pack doesn't feel right, so don't be intimidated by fancy systems and whacky technology...
Hip-Belts And Shoulder Straps: The hip-belt is one of the most crucial parts of the pack. When it works properly, it should help to transfer weight through to the hips and keep the pack stable and comfortable.
Good belts have a hardish outer shell for support with a softer inner layer that conforms to the hips for comfort and support. They are also cupped and relieved to take account of the hip bones and optimise fit.
The better packs also channel the staves of the back system directly into the hip-belt for the most direct possible load transfer, so pay particular attention to how the hip-belt is attached to the rest of the pack. If it's just sort of hanging there, you may not get effective load transfer and could also suffer from poor stability.
Shoulder straps seem to work best when they're contoured ergonomically - think S-shaped - and use a dense to medium dense foam in a medium to narrow strap. You may think soft, wide straps are the way to go, but in practice, they don't offer enough support.
Look too for top-tensioning straps at the very top of the shoulder straps, which allow you to pull the top of the pack in towards your head for increased stability.
Women Specific Packs: If you're a woman, take the time to seek out a women's specific design. Women generally have narrower shoulders, shorter back lengths and wider, more pronounced hips. A well-designed women's pack will take account of all these to produce a pack that's more likely to fit you better.
If a man's pack fits you better, then that's fine, we're all different shapes, but many women will find a women's specific design works better for them. If you're a small bloke, it may be that a women's pack fits you too, so don't get sniffy about it...
Packs and Pockets: You'll find that most large capacity packs have a main compartment - think a very big bag - and possibly a lower 'sleeping bag' section with a removable divider. You don't have to use it for a sleeping bag by the way, it may make more sense to carry a dense, heavy object like a tent there to keep weight low and improve stability.
It's all down to personal preference but compartments make it easier to organise your load rather than just chucking it all in. Large lid pockets offer easy access to frequently used stuff like snacks, hats, gloves and so on.
Side pockets, many fold flat, are similarly handy, but bear in mind that for any sort of mountaineering, a wider pack may get in the way and scrape on rock. For general backpacking though, they're great.
Many packs now have built-in hydration system pockets and outlets - they're fine at the start of the day, but re-filling may require an annoying re-pack of the sack. It may be better to sit the bladder at the top of the pack, in a side pocket or even use water bottles.
Construction: Let's face it, most of us trust the big brands to get it basically right and use proven construction methods and decent fabrics. Look for triple-sewn seams where the fabric of the pack body is sewn together with an overlay of protective tape reinforcing the seam and protecting it from abrasion.
You can tell a bit from the feel of fabrics as well - if it's light and soft feeling, it's probably not going to be as durable as tougher feeling fabric. Finally, look for a reinforced base, either a double fabric layer or a heavier, more durable material as this area takes a real kicking over the years.
On the back system, soft wicking mesh fabrics may work well when new, but bear in mind that they'll wear with use particularly if they pass over the edges of foam pads; look for reinforcement in those areas if durability matters. The same is true of straps.
Waterproofing: Most packs use water resistant material, but will leak through the seams. Some brands use welding and taping techniques to produce waterproof packs - but at a price. Most users will need either a pack cover or (a) waterproof liner(s) to protect their kit in wet conditions.
Going Ultralightweight: If you really, really want to save weight then you can opt for a minimalist pack. Really ultralightweight backpacking is a topic all of its own, but the key to understanding it, particularly if the pack has no back suspension system at all, is that there are no half measures.
If you opt for a pack with no support then you'll also need the lightest sleeping bag, shelter and clothing you can find, plus the nouse to pack them so they don't dig into your back. You also need to bear in mind that while light is great, it's also demanding. Remember too that while lightweight fabrics and materials are proportionately tougher than ever before, they're still less durable than full-weight materials.
We're not saying that you shouldn't aim to lose weight, but bear in mind that well-engineered back systems aren't always the lightest option even if they do offer the best carrying experience with a heavy load.
Gizmos: If you carry trekking poles or ice tools then make sure you have the relevant attachments. Ditto skis, which require reinforced side pockets and straps. If you're going to be climbing, consider a technical sack with gear loops sewn onto the hip-belt.
Types of Pack:
Daypacks: The default daypack used to be around 30 litres in summer and around 40 litres for winter use. These days though, with more and more lightweight gear on the market, it is quite possible to carry pretty much everything you need for a summer day walk or scramble in a 20 or 25-litre pack and use a 25 or 30-litre pack in winter. It really does depend on the kit you have though, and the kind of activity you are going to do.
The majority of 'day' sacks are intended for just that - a day's trip out in the hills. As such, they are much less likely to have much in the way of internal support or complicated back systems. Most of the weight will be suspended from your shoulders, although anything above about 20 litres in size is likely to come with a waist strap, so you can stabilise the pack, and distribute some of the weight off your shoulders and onto your hips.
The main things to look for are comfort and stability when it is fully loaded, and then whether the pack has the type, size and number of 'extras' you need such as pockets, hydration system compatibility, padded back and straps for fixing trekking poles, ice axes etc
Alpine/Climbing Packs: These are generally 35-55 litres in capacity - to enable you to carry all the extra hardware you need for your trip. Generally they will have some internal support, and a more substantial hip belt/shoulder straps to help you carry your load more comfortably. Hipbelts on some Alpine packs may be detachable, or have gear loops stitched on. These packs tend to be taller and slimmer with one main compartment, a lid pocket and minimal or no side pockets, so they do not get in the way. They will also be likely to have side compression straps to adjust the size and volume of your load at different points throughout the trip.
Backpacking/Trekking Packs: Much larger packs, these are generally between 55-80 litres in size, specifically designed to carry heavier, multi-day loads. The internal frame gives rigidity and structure, and coupled with a sturdy, padded back system (often adjustable) will distribute the load between your shoulders, hips and back. You are likely to find a lower compartment that can be used to divide and organise your load if wanted and side pockets are usually large enough to store most things you will need during the day (water bottles, snacks, hat/gloves etc) without having to go rummaging around in your main pack for them.
Travel Packs: These have come on in leaps and bounds over the last 10 years. They range in size, but are usually around the 50 litre mark. The main advantage is that the compartments zip open fully, opening the pack like a suitcase and allowing for easier packing and retrieving of your gear. This type of pack will have a concealable back system and probably some sort of lockable zip system as well, for more secure airport travel. They may well also have wheels and extendable handles. Some of the larger versions also include a small daypack that can be zipped on and off the main travel pack - giving you ultimate flexibility for day trips or hand luggage.