Remember the last time you realized you were charged for something only after looking at your credit card bill? Well, you’re not alone.

Thirty-five percent of Americans say they signed up for a subscription or membership, such as a gym, that enrolled them in an automatic payment plan without even realizing it, according to a new survey of 1,002 adults by CreditCards.com.

Many businesses, from media to online retailers, offer free trials to people who provide their credit card information. But when the free lunch comes to an end, it’s easy for people to forget to cancel it in time even if they don’t want to continue the service.

The survey found that Gen-Xers (44%) and millennials (37%) were most likely to accidentally sign up for automatic payments. Meanwhile, the Silent Generation (age 72 and older) were more likely than any other age group to say they had never signed up for automatic payments without knowing it.

Monitor your credit card bills to avoid recurring payments

While it’s common for internet companies like Amazon (AMZN) to offer free trials, 48% of respondents say they have forgotten to cancel a free trial before being charged by an auto-renewal.

Some consumers have expressed their frustration over this phenomenon on social media. “So apparently I started a ‘free’ Netflix (NFLX) account in 2014 while TDY and forgot to cancel it. $300 later I figured it out… DUMB!” One Facebook user wrote.

One simple way to avoid regrets is to schedule time in your calendar as a reminder to cancel the subscription before the trial period ends, says Matt Schulz, CreditCards.com’s senior industry analyst.

Schulz thinks it’s also important for users to keep an eye on their bank statements every week. “Doing so can help you spot forgotten recurring charges when they pop up. It doesn’t take much time, but it’s very important,” he said.

Besides online banks, some third-party mobile apps also help people identify recurring payments. Mint, for example, is a personal financial management service that helps categorize expenses to spot changes. Apps like Truebill and Trims monitor paid subscriptions from your credit card records and help cancel unwanted ones.

Canceling may be hard, so be persistent

While signing up for new services may only take a click, getting off the hook may be not. Only about half of those surveyed found it easy to cancel an automatic payment. Sometimes it takes an email or a lengthy call with customer service.

Jessica Alba’s The Honest Company, which sells eco-friendly products to parents, for example, was once criticized for being difficult to unsubscribe to. The company required customers to cancel by phone, and one Gizmodo reporter waited a half-hour to speak to a human being when calling the company’s help line. After consumers filed complaints with the Federal Trade Commission, the company updated its policy to enable online cancellation.

“Be proactive and ask about it when you’re signing up. That’s likely when the company will be most responsive to your questions,” Schulz says. He suggests people approach the company through social media or leave comments on a reviews website if the canceling process brings too many headaches. “Make sure that you are heard,” he said, “and they’ll react to you.”

Krystal Hu is a reporter at Yahoo Finance. Follow her on Twitter.

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