“I was like, you can’t do that,” Ewen told Fortune. Not because Amex points can’t be used on Amazon (they can) but because using them that way destroys their value. Transferred to the right airline partner, those same points could have covered a round-trip flight. Instead, they bought an appliance at a redemption rate that valued each point at less than a cent.

It’s the kind of mistake that happens when someone collects all their points in one place and never learns what they’re actually worth, or what they can be used for.

Ewen has spent two decades in the points and miles space, and one of the principles he returns to most often is diversification. Not just of cards, but of the currencies they earn.

“It’s just like an investor strategy,” he says. “You don’t want to be all in on one stock, because if that stock tanks, you’re going to be left out in the cold.”

The same logic applies to loyalty programs. If you’ve put every dollar of spend toward Delta SkyMiles and then Delta devalues its award chart, raises redemption prices, or eliminates a route you rely on, you’re stuck. You have a pile of currency that just lost purchasing power and have no backup.

“If you are fully in on Delta SkyMiles and then Delta changes something that you don’t like, that doesn’t give you a ton of flexibility,” Ewen says. “Whereas, if you have some miles with Delta, some with United, some with Chase, that allows you to be protected from some of those changes.”

The points comparison to portfolio management isn’t just a metaphor. Airline loyalty programs are now valued in the tens of billions of dollars—in some cases worth more than the airlines themselves. During the pandemic, United, Delta, and American collectively raised $26 billion in debt backed by their frequent flyer programs. United’s MileagePlus alone was appraised at $22 billion, more than double the airline’s equity value at the time. American’s AAdvantage was valued at up to $30 billion while the airline itself was worth less than $7 billion. When there’s a points program that large that adjusts its pricing, the ripple effects hit millions of point balances simultaneously. That’s why Ewen said diversification is the hedge—but it comes with its own negatives, too.

“There is such a thing as being spread too thinly and being too diversified, especially if you are not spending a ton of money from month to month,” he said. “It’s harder to kind of generate significant balances of points.” The person spending $3,000 a month across four different programs may never accumulate enough in any one of them to book anything meaningful. The person concentrating $3,000 on two well-chosen programs has a better shot at a real redemption.

The right approach is somewhere in the middle, and it starts with matching the card to what you actually spend on, not what an influencer told you to sign up for. “The first thing is, what are you trying to do with this?” Ewen asked. “If someone says, ‘I don’t know, maybe a trip, I only travel about once a year, it’s normally a road trip’—great, a cash back card is going to be the best fit.”

How should I spend my points, then?

For people who do travel enough to justify a travel card, Ewen recommends starting with a flexible points currency (like Chase Ultimate Rewards, Amex Membership Rewards, or Capital One miles) rather than a co-branded airline or hotel card. A flexible currency can transfer to multiple airline and hotel partners, meaning you’re not locked into one program’s pricing.

Take, for example, United credit cards, issued by Chase and popular with loyal United flyers. But the Chase Sapphire Preferred, which has a lower annual fee, actually earns at better rates on dining, general travel, and online groceries. And because Chase points transfer to United at a one-to-one ratio, you can end up with more United miles through the Sapphire Preferred than you would through the United card itself. Plus, you retain the flexibility to send those points to Hyatt, Southwest, or any other Chase transfer partner if United’s pricing doesn’t work for a given trip.

“There are, weirdly, oftentimes much better options than having a co-branded credit card,” Ewen said.

Richard Kerr, GM of Travel at Bilt, sees the same dynamic from the issuer side. The co-branded landscape has become so crowded consumers face decision fatigue before they even start optimizing.

“It’s now an incredibly competitive world,” Kerr says. “Not only a million options between different airlines and hotels, but each airline and hotel has four or five different options for you to take a look at.”

More cards means more annual fees, more interchange revenue, and more opportunities to lock a customer into a single ecosystem. For consumers, the antidote is the same one any financial advisor would give about an investment portfolio: spread your risk, know what you own, and don’t chase performance.

“Start with one. Get comfortable with that,” Ewen said for anyone looking to get into the points game. “And then if you add a second one with maybe a couple different bonus categories, have that for six months or a year. Make it part of your muscle memory.”

His wife is the proof of concept: She went from needing a handwritten cheat sheet to knowing instinctively which card to pull at the grocery store, the gas station, and the restaurant.

“It just became part of her muscle memory,” he said. “It took time to get there, but it’s important to not bite off more than you can chew.”

For everyone else—the person who doesn’t want a cheat sheet, who doesn’t want to track quarterly bonus categories, who just wants to stop leaving money on the table without making it a second job— both Ewen and Kerr arrive at the same place.

“Never a wrong way to go,” Kerr says of a no-annual-fee card with a flat 2% cash back on everything.