Backpack - 65l minimum - We had a couple of scouts take 45l packs and they ended up looking like the Beverly Hillbillies moving with all the gear hanging off outside. Their packs were unbalanced, snagged on trees, and things kept falling off. 65 liters is a minimum unless you can afford all the expensive and small ultralight gear.
Osprey Atmos 65 - The Atmos is popular with our crew due to comfort, features, lightweight, and a good balance between features and cost. Features we love about the Atmos 65 include:
VERY comfortable carrying and a breathable back
Gadget space - belt pockets, side water bottle storage accessible from front or top, detachable lid with two pockets, trekking pole holder, etc.
Lifetime guarantee on Osprey packs
Available in S, M and L depending on your body size
If the Atmos doesn't have the features you want, the Osprey website has an awesome Packfinder tool that will help you select a pack based on the features you like, size needed, type of use, price, packs for women, etc.
Lightweight and just large enough to carry the essentials (water, food, first aid, rain gear, compass, knife, matches/lighter, etc).
Some backpacks have a detachable lid that works as a daypack or a daypack that is made to specifically attach to your backpack
Cheap but held up great. About half of our troop used these packs. They're so popular that Tooth of Time Traders carried them in their brick and mortar store at Philmont.
Lashing straps - Keep gear on the exterior of your pack to a minimum. Things outside the pack fall off, get wet and dirty, snag on trees and rocks, and make it difficult to keep your rain cover on. That said, if you do need to attach a sleeping bag, bedroll or other item to the outside of your pack, make sure you have straps that will hold the items securely and have a wide range of adjustment. At a bare minimum, carry some paracord to lash unexpected items to the outside of your pack.
Gallon Ziploc bags 6-12 - When they tell you August is the rainy season at Philmont, believe them. 9 out of 10 days it rained on us. Anything that helps keep your gear dry is a must.
Pack cover - I didn't take a pack cover. My Osprey Atmos says "Water Resistant". It was resistant for the first part of the trek it was fine. Day four I'm breaking camp on Mount Baldy and I dumped over a half gallon of water out of my pack. I decided I would invest in a pack cover next time.
Small stuff sacks - Dry bag stuff sacks are awesome. One more layer of protection against water. Use different colors so you can quickly find what you're looking for - eg - sleeping clothes in the yellow bag, smellable clothes in the blue bag.
Alps Mountaineering Dry Bag Stuff Sacks - Great for an added layer of water resistance that will still compress the contents. I pack smaller dry bags inside these to help keep items separated. I use a medium size to protect my sleeping bag. They provide pretty good water protection but will eventually have some seepage in very wet scenarios - don't use these for a canoe trek.
Ditty Bags - These can be almost anything. I like to use small dry bags to make sure things stay dry.