Your Weight and Your Fate
Chiropractor Isis M. Medina's rule is that you should tote no more than 10 percent of your body weight—so a bag that's more than 5 pounds when empty is a bad start. If your load is excessive, your head and neck jut forward rather than staying over your shoulders, which can lead to headaches, neck tension, and back pain.

Getting a Handle
Long-strapped shoulder bags are out: They tug you to one side in a C shape, with the purse bumping at your hip and the strap slipping (so you hunch up your shoulder to keep it in place). Avoid chains too; they tend to gouge the flesh. Look for short handles, not too flimsy or narrow (with very skinny straps on a hefty bag, you're asking for trouble).

The Test-Drive
You wouldn't buy a dress without trying it on; follow that rule with a prospective pocketbook. Medina's three approved positions for a short-handled bag: over your shoulder and tucked under your arm; over your forearm (or in the crook of your elbow); or held in your hand. When worn over the shoulder, your bag should fall as close to your center of gravity as possible (around the navel or waist, on average; a little higher—at chest level—if you're petite). Very busty women should avoid carrying anything on the shoulder: They're already being pulled forward by the weight of their breasts, and this aggravates the misalignment.

Compact Models
An oversize bag is dangerous because it invites you to carry tons of stuff, but it isn't intrinsically worse than a small one. Shape and dimensions matter less than how the bag meshes with your body, the doctor says. If it's on your shoulder, it should automatically find the sweet spot between your side and arm. If it's over your forearm, it should be right at your waist (which means women with long torsos or a lot of height can afford a deeper bag).

No Feed Bags
Slouchy may be in, but Medina likes structure: When items shift around and aren't well distributed, your balance is thrown off. Even an oversize bag can work, the doctor assures us, if it's got a definite shape, a firm base, and inner compartments to keep belongings stable.

Easy Access
If you have to do a major dig to find your phone or comb, you're twisting and distorting your torso. You want possessions to be reachable yet secure—a long top zipper or magnetic closure helps; Medina also likes outside pockets for frequently needed essentials.

Low-Bulk Components
Best are bags that are fairly flat and of soft, lightweight material that molds to the body—buttery leather is great, but so is nylon, Medina says. Beware of stiff leather ("It doesn't feel friendly"); chunky hardware; big studs, stones, or other doodads that keep you and your bag apart. A fat purse on your shoulder will force your arm to stick out rather than staying comfortably at your side.

Beyond Labels
Medina checked out hundreds of styles without knowing the cost or designer—a sort of blind bag test. "I was really impressed by some of the inexpensive ones," she says. Not to knock luxury, but a modestly priced, well-made bag can be totally back healthy and astonishingly chic.

Can You Be Too Sensible?
Ugly stepsisters of the bag family—hideous but anatomically correct—are best worn on a moonless night. Fanny packs work, says Medina, because they rest on the lower lumbar vertebrae—large bones intended to bear more weight than the neck and upper back. Use backpacks to transport something heavy, like a laptop, but carry them low and keep your chest open. Flat, kidney-shaped ergonomic bags meet the doctor's medical standards but offend her fashion sense: "I guess it's the girl in me."

And How Heavy Is Your Bag?
O piled the things we routinely carry on a scale (total: 15 pounds). Add substantial extras like a laptop, water bottle, or pair of shoes for evening, then factor in a bag that's heavy from the get-go, and you're in trouble. (Do the math: A 140-pound woman should carry 14 pounds at most, according to Medina's 10 percent formula.) Watch, too, that your whole load isn't on the same side: The doctor recommends carrying a bag on one shoulder and a short-handled tote in the other hand, so you're balanced.