Together with your footwear, your pack is the thing that will make the most difference to your comfort levels while tramping.
What you need in a pack:
At least 50L, I recommend 65L as this is the most versatile size. Some people who carry more stuff or are taller than average may struggle to fit everything into a 50L. That said, if you are a smaller person 50L may be plenty. That is if you only want to buy one pack, most more experienced hikers will often have at least a couple of packs of different sizes, smaller ones for smaller trips and larger ones for longer trips. For example, I use a 33L for most trips, but I have a 90L for longer trips.
A decent harness – the harness is the shoulder straps, hip belt and everything that holds it together. It is the single most important feature of any pack.
A hip-belt that fits!! This is the number one problem people, particularly younger people, have with their packs. Harnesses nowadays are almost all designed to transfer as much weight to the hips as possible. If the hip-belt is too large all the weight will be on the shoulders, which is very uncomfortable. Ideally, you should not be able to tighten the hip-belt all the way.
Top entry – so the pack opens at the top.
Nice features to have:
An adjustable back harness, fixed length harnesses are "one-size fits all" but rarely fit anyone well, better to be able to adjust it.
Some outside pockets – although not too many.
Some type of ventilated back – stops your back getting sweaty, won’t work perfectly, but definitely helps. Although be wary of packs that have a massive curve in the back as this negatively affects the weight distribution.
Some external straps to attach things – although again not too many.
Lightweight – don’t go buying a pack over 2.5kg at 65L, beyond this is too heavy for a modern pack.
General buying advice:
Shop around, try on lots of different packs, not just within the same brand.
My advice is to get something around 65L if you are of average size, if you're smaller than that get a 50L, if you're quite tall (above 6’3), maybe a 70L or 75L.
Make sure the salesperson knows what they’re talking about and fits it to you properly.
Don’t buy any pack marked ‘travel pack’ or something similar. These are not usually overly good for hiking. They are usually designed with a sub standard harness, and with many features that just add weight without any added functionality for hiking. This being said, if you already have such a pack don’t go out and buy a new one if you’re not sure you want to get into hiking.
There are so many different configurations of packs, it really does come down to personal preferences. Unlike rainwear, there is not such a clear relationship between price and quality. It really all comes down to the design of the harness, everything beyond that (material, pockets, size) is just personal preference.
Here are three recommendations representing perhaps three levels of price and quality; as with rainwear, there are many, many packs out there and these are only three that I am familiar with. All of these have 50L variants. These are all classic backpacking packs, in that they can all take heavy loads, and have a lot of space. However, if you are looking to go down the lightweight hiking route, these packs are not for you, as they are all fairly heavy.
Macpac Torlesse 65 - Lots of external storage, forgiving harness (by which I mean not terribly well desgined, but makes up for it with lots of padding), relatively cheap on sale. But if you can spend a bit more, I recommend going for something like the ones below.
Osprey Atmos AG 65 – Lots of external storage, very cool harness design, designed to hug the whole body, made of lightweight, durable fabric. I haven’t actually owned one, but I’ve heard very good things.
Aarn Effortless Rhythm – One of the most innovative pack designs around, extremely comfortable, lightweight and very configurable to the individual; what I use now for longer trips.
Something to remember: When you look at the full retail price of these packs (rrp), you will see that they are not that different. However, the trick comes in looking at their sale prices. Macpac packs become significantly discounted sometimes by more than 40%, Bivoauc packs sometimes come down by as much as 30%, but Aarn packs will rarely have more than 10% off. Recall what I said on the buying tips main page about the perils of comparing full retail prices on outdoor gear.
A pack liner is a big bag that goes inside your pack that keeps everything else dry. Very few packs are truly waterproof, a pack liner is the way to prevent your things getting wet; it is very important. The cheapest option is a couple of trash bags; however for around $5 you can buy a pack liner from most good outdoor stores, its essentially a big yellow thick plastic bag; this is what most trampers use. However, if you are intending to submerge your pack frequently, a proper dry bag pack liner is the only way to go, though these are much more expensive.
A pack cover is a piece of waterproof material that goes over your pack. It keeps your pack clean and keeps most water out, although it is not a replacement for a pack liner. The advantage of keeping your pack dry is that when it is wet it will be heavier from all of the water it has absorbed. It also keeps it clean. However, the cover itself is also added weight, so it comes down to personal preference whether to use one or not.