Pack Mechanics
Internal frames conceal the bones of the pack inside a nylon shell, hug your torso, and carry the load low and close to the back. They are more stable for activities that demand freedom of movement, like rugged hiking and climbing. External-frame packs allow air to circulate between the frame and your back, keeping you cooler. Their higher center of gravity transfers more weight to your hips, allowing an upright gait and good stability on trails. They typically have multiple pockets and cost less than internals.

Most packs, whether internal- or external-frame, are top-loaders — meaning that except for a sleeping bag compartment, you load them from the top. Panel-loading packs have a long, vertical zipper along one or both sides for access to the main compartment; some have a top opening as well. With top-loaders, digging for something buried inside becomes a chore. With a panel-loader, contents can shift around once you open that side zipper, requiring repacking. Also, zippers are potential places for water to get inside your pack. (A waterproof pack cover alleviates that concern, and being lightweight and only $15 to $25, it should be standard gear in the Northeast. Or simply line your pack with a thick plastic bag.)

Finding your Fit
Comfort is determined by a pack's suspension system — i.e., hip belt, shoulder straps, and connected parts. Suspension systems come in many incarnations and differ in how well they distribute the load and how precisely they can be adjusted to fit. Most packs come in two or three sizes of suspension system, and parts like the hip belt can often be swapped out for a better fit. Women's packs have suspensions designed for their shorter torsos and wider hips.

Suspension system sizes correspond to torso measurements. To measure your torso, stand at attention and have someone extend a tape measure from your seventh vertebra — the prominent bone at the base of your neck — along your spine to the top of your hip bones, which you can find by placing your hands on your hips and drawing a line between your thumbs.

Packs are like boots — you may have to try on several models to get a good fit. When trying one on, fill it with the stuff you'll carry on the trail — no more than one-quarter to one-third of your body weight is the general guideline. Loosen all the suspension straps. Buckle the belt and chest strap. With the hip belt resting atop your hip bones, not sliding down over them, pull the belt comfortably snug. The chest strap should sit about three inches below your collarbone, though women might prefer it a little higher.

Tighten the stabilizer straps, beginning with the lowest one. Next, tighten the load-lifters, which should lie at a 45-degree angle to your shoulders. Lastly, tighten the shoulder straps and chest strap. You should have leeway in the belt and all straps to loosen or tighten further. The shoulder straps should wrap cleanly around your shoulders, without gaps or bunching, and extend about a hand's width beneath your armpits; if they don't, try adjusting the shoulder yoke (behind the back pad) up or down.

When properly adjusted, about two-thirds of the pack's weight should ride on your hips. Walk around the store wearing your loaded pack. If it doesn't feel comfortable, find another one.

Today's Features
Beyond fit and capacity, you'll choose a pack based on its features. Some are Spartan, others have more bells and whistles than a Lexus. These things make a pack more functional but also increase its weight and cost. Ideally, you find a pack with features you want without paying for extras you don't need.

Some standard features include horizontal compression straps to prevent contents from shifting; daisy chains, or nylon loops, for lashing things to the pack; and ice ax loops. Mesh bottle pockets or hydration systems — internal bladders with external sipping hoses — provide easy access to water. Some packs have a detachable top pocket for use as a day pack. Think about whether you want external pockets to keep items handy, or whether they'll get in the way — for instance, bulky side pockets aren't very useful to someone planning to carry skis on the pack.

Planning to spend a lot on a pack? Then settle for nothing less than a perfect fit and all the features you want. At the other end of the spectrum, the least-expensive packs will sacrifice some comfort and features. But even with a bargain pack, search for a good fit and check for solid construction — durable pack cloth, tough seams, decent padding in the hip belt and shoulder straps. In the middle price range, you'll find packs that adjust to various torso sizes.

Remember: It shouldn't hurt to be a backpacker.

—Michael Lanza is author of The Ultimate Guide to Backcountry Travel, from AMC Books.