Attention must be given to the weight of necessary items that you MUST carry to safely complete your mission, your physical condition, and your need for mobility. The approach used by search and rescue groups and highly recommended is a total package consisting of three LEVELS:
Level I: Your "Every Day Carry" or "EDC" that you always have with you on your key ring, in pockets, etc.
Level II: Is the minimum "grab and go" items needed to conduct outdoor activity, away from your vehicle, for a 12-hour period. This compact survival kit should include personal medications, snacks and a water bottle and the "ten essentials " It can be carried easily in a lumbar pack, multi-pocket vest, or zippered pouch which fits in your briefcase. The most popular waist pack among professional EMTs, ski patrol and SAR operators is the Harper: http://www.harperpack.com/comp.html
Level III: Expands supplements the first two levels to include additional supplies of expendables, like AA batteries for flashlights, plus adequate food, hunting and foraging items, water, containers and purification, extra clothing, shelter, tools, navigation, signalling and first aid items to enable you to survive in safety and relative comfort for not less than 72 hours. This is your "survival ruck" or "SR." Many operators carry their 2-way radio and Level II gear in a utility pouch, attached to the SR with a snap-link so that if necessary they can "drop rucks" and still take the essentials with them. There are LOTS of options among the three general types of PACKS.
• Unframed packs are useful for storing loads loads transpoorted primarily in a vehicle, and carried on your back for relative short distances on moderate terrain, as in portages. The Duluth type is the best example. Duluth Pack
• Internal frame packs are standard in the SAR community and ride close to the body. They have fewer projections which catch or snag on brush, have external zippered compartments to organize loads, and usually compression straps to adjust for various size loads and sometimes external attachment points for carrying climbing or technical rescue gear. Search and Rescue Pack - Coaxsher SR-1 Endeavor
Keep in mind that many of us reading this forum are "middle aged" and older and not in the prime of physical condition which we once were. A pack weighing 25 to 30 pounds is the maximum which a 40+ to 60 year-old male in "good" physical condition, with no medical problems, should carry all day on easy terrain. Anyone over age 50 should first seek medical evaluation before undertaking any strenuous activity, followed up when deemed medically necessary by a stress test and structured conditioning program, under medical supervision.
For the Canadians on the forum - a membership to Mountain Equipment Co-Op is almost essential. They have all the gear that you need for repelling, hiking, snow-shoeing, x-country skiing, camping, etc. They have some great designs for back-packs (I have a couple - one for a week of camping and one for day-trips) and fanny-packs (perfect for a hike or x-country ski).
I combine those with take-down x-country ski-poles (hiking poles, snow-shoe poles, multi-use poles) that strap to the packs easily. They also have system to strap on a climbing axe, cramp-ons, etc.
Membership to MEC is cheap - and the equipment is top-notch!!
In the states, REI has a great assortment of packs and day packs. Most of their employees are somewhat knowledgeable as well. I have used severl different brands - Kelty and Northface have always been good quality for me.
One thing you really need to do is go try some on. Each pack is going to fit a bit different, also if you can have them put in about the weight you will be hauling around this will be good as well, any good camping store will have weights around you can add to the packs
I would say once you know what you are looking for check out the deals at Campmor.com I picked up a Jansport Big Bear 5000 cu in for 40.00
Currently I bought a new backpack for my camping trips:
'The North Face Akila 55 Backpack - Women's - 2750cu in'
I think if you go hiking and camping it is very important to have a good backpack in order to avoid serious damage to your back. I am very satisfied with this product. Although it is expensive I am of the opinion that it was a very good investment.
If you're Canadian just to got Mountain Equipment Co-Op and they'll help you pick one and fit it properly. I like the MEC packs and Arcteryx Packs. I never liked the Gregory packs. I don't like the mounting system and they sit really high on me. But packs fit every body differently. I find Gregory packs are fine if you're skinny and tall. I'm kinda short.
A day hiking pack should be pretty small. I used to use a military harness thing with a water bladder on the back two extra containers of water and I'd fill all the compartments with my other gear. Great if you have to climb over something.
If you have to do any climbing keep your pack small and light. I've climbed with a fully loaded 75L pack on and it's real scary!
I'd assemble all your gear for your hike on the kitchen table and then figure out how big a pack you need based on how much gear you're putting in. A "just right" pack is always best. If you underload a huge pack it won't fit right and it'll just get in the way.
I'm partial to Osprey packs. Lots of my SAR mates use them. They hold up well, and the ones I've used are very comfortable as long as you size and adjust them correctly and don't overload. Good gear accessibility too.
Just my two cents.
I used an external frame Kelty Tioga on my A.T. thru-hike and, though I went the distance with it, I hung it up after that. (Literally.) It was squeaky, for one, but the bigger issue for me was that the frame made it difficult for me to negotiate rough terrain. The thing would get hung up on branches at times, and if I wanted to sit down and lower myself face-foward, the frame forced me to lean forward ... Not always to my liking. On the plus side, externals are usually great for organizing gear.
I like internal frames. I have several Lowe Alpine packs that I have used from the Sierra Nevada range across the US including the Rockies and Smokey Mountains.
Backpacks are like cars, you have to find the one that fits you. Any of the above named packs are great ones, you need to go someplace that will let you load down a pack (most places have weights that you can put in them) and try it out in the store-walking up and down stairs and such. Remember to have them explain ALL of the straps and mountings to you, each strap and mounting does something different to lighten your load on the back. Up hills pull this strap, down hill tighten this one, loosen that one... it's a science really.
The best part of getting a new pack is trying out different ones.
Obviously the size of your backpack and whether it has a frame depends on what you think you use it for. If you want it for a BOB then you might just want something smaller that you can grab easily and perhaps run with. There are some simple backpacks at Emergency Prepared Backpacks. I even have a great sturdy backpack that I purchased from Smith's. There are cheap AND reliable bags out there. Don't get too caught up in assuring quality by buying high-priced backpack unless you expect to give it some good use to get your money's worth out of it. For just a BOB or survival kit, I would recommend a simple yet sturdy bag that is reasonably priced(around $15-$35).
I forgot about the Osprey packs. Back in the day they were hard to find in Canada. All my buddies at work wanted one and they arranged a trade of some gear with an American sales rep to get a bunch. They were really nice packs.
If you need to carry a volkswagen on your back don't forget about external frame packs. Overkill for most folks but for ultra heavy loads nothing beats it.
I swear by my Gregory Whitney. It's a 98L pack, so big enough for most any long backpacking trip, but if I'm only going for an overnighter I can cinch it in and only use the space I need. Of all the packs I looked at (and I looked at many) it was the one with the best-padded and most comfortable hip belt at the time I bought it. On the West Coast Trail I had to stock up on one day & night's water for our whole group of 6, and hike it for about 10km. I figure for those 10km, I was up to about 90 lbs, and it felt totally fine on my back.
But like others have said, you gotta try them on to find the right one for you.
I have several packs mostly Dana Designs, they range in size from 2,000-6,000 cubic inches. They are all internal frame packs, they will help keep you warm in a cold area. For desert climates use an external frame pack, they will keep you cooler. For areas of heavy undergrowth use an internal frame pack, they are closer to your body and cause less obstruction. When I first got back into backing as an adult the out fitter saw me comming. I bought a 6,000 cubic inch pack and all kinds of bulky or heavy stuff to put into it. Over the years I have become a minimalist and can get by with 3,000 to 4,000 cubic inches. There used to be a site called Ron Jardines 28# pack. His equiptment and food for a week only weighed 28#'s. I haven't seen it in years. I would also recomend look at some Applachian Trail Hiking sites or blogs. They are full of good info. One of the guys I work with in scouts has been hiking the trail for years and is loaded with good info.