What’s the one piece of workout gear you can’t live without? Your Ipod Nano? A good water bottle? A truly supportive sports bra?

Wrong, wrong, and wrong. The single most important piece of equipment to virtually any kind of exercise program — running, aerobics, hiking, tennis, basketball — is the right pair of shoes.

A good pair of shoes can make or break your workout — but it’s easy to go wrong.

1. Grabbing Whatever’s Handy

“The biggest mistake people make when they start running, jogging, or doing any exercise program, is just reaching into the closet and pulling out an old pair of sneakers,” says Tracie Rodgers, PhD, an exercise psychologist and spokeswoman for the American Council on Exercise.

But how do you choose the right shoe for your workout?

A recent search of a popular shoe-buying web site yielded more than 4,500 different pairs under the category “sneakers,” including more than 1,000 running shoes, 199 “cross-trainers,” 133 pairs of basketball shoes, 110 pairs for tennis, and more than 1,500 in a nebulous category dubbed “athleisure.” Supposedly, you can wear these to the office and for a workout — but you probably shouldn’t.

2. Choosing the Right Shoe — for the Wrong Workout

First, you need to choose the right type of shoe for the kind of workout you’ll be doing. And yes, it does matter.

A shoe made for running is very different from a shoe made for basketball or tennis, in a number of ways.

“Running shoes have no lateral stability built into them, because you don’t move your feet laterally when you run,” says Joe Puleo, the author of Running Anatomy and the head men’s and women’s cross-country and track and field coach at Rutgers University in Camden, N.J.

“You’re only going forward, and a running shoe is built to give you support and stability as you move your foot through the running gait cycle,” Puleo says. “Basketball and tennis shoes both have to be stabilized laterally, because you move your feet side to side a lot when playing these sports. You can’t build a running shoe that has lateral stability, and you can’t build a shoe for basketball or tennis that doesn’t have it.”

Even walking shoes differ from running shoes.

“Runners land more on their forefoot, while when walking you have a heavier heel strike,” says Catherine Cheung, DPM, a podiatrist and foot surgeon with the Post Street Surgery Center in San Francisco. “So for running, you want a shoe that has more cushioning on the forefoot, while walking shoes should have stiffer rubber to support the heel.”

Can’t you just get a good cross-trainer and use it for everything? Probably not.

“Cross-trainer” shoes never existed before Bo Jackson, who played professional baseball and football (remember the “Bo Knows” ad campaign?).

“Before Jackson, we just called them sneakers,” Puleo says. “Then, Nike came up with an ad campaign and now we have cross-trainers. But there’s no specificity to them: you can’t do any one thing well. They have some lateral stability, so you can play a game of basketball with your kids occasionally. You can run a mile or two. But most of them are not very good shoes for any particular activity.”

Then again, some people aren’t heavily into running, hiking, tennis, or any one sport. They go to the gym occasionally, maybe play tennis with a work buddy once in a while, or shoot a few baskets with the kids.

For them, a cross-trainer might be the best choice.

“A good cross-trainer will allow you to do the treadmill, some walking on asphalt or on a track, and light jogging,” says Kathleen Stone, DPM, president of the American Podiatric Medical Association. “Not mileage, of course. But I like them for people who are doing a variety of athletic endeavors casually.”

To choose a good cross-trainer, Stone suggests you look for:

A firm heel
Good support (you shouldn’t be able to bend the shoe too easily)
Light weight (you don’t want to add a lot of pounds to your feet)

But the APMA recommends that if you’re going to participate in a particular sport on a regular basis (2-3 times a week or more), you should choose a sport-specific shoe.

3. Loving Them Too Much

“Your workout shoes should be your workout shoes, and not your running-around-town shoes,” Rodgers says. “You’ll break down a pair of shoes standing in them or wearing them to the mall and running errands much faster than when you’re running or exercising.”

So buy yourself a pair of casual tennies for running around town, and stow your good workout shoes in the closet as soon as you get home from your run or your tennis game.

“That’s where I buy the shoes I think look nice, but aren’t good for me to work out in,” Rodgers says. “Certain brands, I can’t work out in because they hurt my feet, but I love the way they look, so I wear them with my jeans for just hanging around.”

4. Loving Them Too Long

Another big mistake many people make when buying athletic shoes is not replacing them often enough.

“They think they should replace their workout shoes when they start looking bad,” Rodgers says. “But shoes start to break down while they’re still looking good. The support — the reason you buy the shoe in the first place — is gone, and you’ll start feeling strange aches and pains in your knees, hip, and back.”

Most experts recommend that runners replace their shoes every 300-500 miles. If you don’t run enough to have a mile count, or running’s not your sport, you should replace your athletic shoes at least once a year.

“If you’re exercising on a casual basis, you can make your shoes last a year, but if you’re working out every day, six months is pretty much your limit,” Stone says.

You should also have your shoe size rechecked every year, Cheung says. “Foot size doesn’t stay the same; our feet tend to grow bigger as we age.”

Do you need orthotics — the specialized, custom-built inserts designed for people with specific gait problems? For most people, the answer is probably no, Puleo says.

“There are certain foot types and injuries that can be corrected with orthotics, but my opinion is they’re dramatically overprescribed. They work well for some patients, but I’ve been wearing the same brand of over-the-counter generic insoles for years, and they’ve worked great as well, and are much cheaper,” Puleo says.

5. Doing It Yourself

Unless you’ve been playing your sport for a long time and have learned exactly what shoe is right for you, it’s a bad idea to just walk into a sporting goods store, try on a few pairs of shoes, and walk out with what you think is best.

Instead, go to an athletic shoe specialty store to get an expert insight on the right shoe and the best fit.

“The staff there will do a real fitting, evaluate your foot, and take a history of your athletic activities and what shoes may have worked for you before,” Puleo says. “They’ll watch you walk or run on a treadmill or outside.”

And they’ll take three measurements — not just one — on the metal plate we’ve all seen in shoe stores, known as a Brannock device.

“You need to know not just length, but also width and arch length,” Puleo says. “All three of those numbers together determine what size you should wear. And each shoe can be cut a little differently — a 10.5 isn’t a universal 10.5 in all shoes — so they’ll start with that number and work from there.”

A good athletic shoe specialty store will also have a liberal return policy — so ask. Others may permit you to return shoes if you’ve only worn them indoors, but not outdoors.

The New Jersey running store Puleo founded allowed customers to return a shoe at any time, for any reason. “You don’t like ‘em, you bring ‘em back,” he says. “It was on me to make sure you were satisfied before you left. We had a very low rate of return because we spent so much time with every customer, we knew they’d be happy with them. You should never be stuck with a shoe that doesn’t work for you.”