Here’s a suggestion for those planning to make big changes in the new year: Consider making them small instead.

This approach is likely to be especially helpful in 2021, after a year when many have been under financial strain. People who are stressed or overwhelmed, “cannot make big changes and aren’t likely to even try,” said BJ Fogg, a behavior scientist at Stanford University.

To boost the odds of success, he recommends focusing on small, even tiny, changes—such as doing two push-ups a day or saving 1% of pay.

The easier the task, the higher the odds that you will keep it going when motivation inevitably flags, said Dr. Fogg, author of “Tiny Habits: The Small Changes that Change Everything.” When a small habit becomes ingrained, it creates a foundation for bigger habits—for example, doing six push-ups a day or saving 2% of pay.

Here are strategies to consider.

Don’t rely on motivation

New Year’s is when many people feel motivated to make changes, including making saving for retirement a priority. But motivation can dissipate quickly, as anyone who has joined a gym in January and stopped going in February knows.

“Motivation may suffice for one-time feats but

it’s not enough for sustained change,” said Dr. Fogg, who instead recommends finding ways to shrink your goals to make them easier to accomplish.

Keep the bar low

Dr. Fogg says people often set themselves up for failure by choosing goals that are vague or overly ambitious, such as achieving financial security.

The key is to attach such goals to specific actions, such as establishing an emergency fund or catching up on retirement savings, and to start with a realistic step, said Dan Egan, managing director of behavioral finance and investing at Betterment LLC. For example, you might decide to save $500 in an emergency account or 1% of pay in your 401(k).

By sticking with a 1% savings rate, you are more likely to keep the habit alive when an unexpected expense arises, Dr. Fogg said.

Stress the positive

People have a tendency to focus on the negative. But resist the urge to dwell on how far behind you are or beat yourself up for past missteps, such as cashing out a 401(k) when leaving a job.

Instead, focus on how the small steps you are taking now will add up to something substantial over time, said Ramit Sethi, author of “I Will Teach You to Be Rich.”

“One reason we are hesitant to make small changes is that they seem so ineffective and minor,” he said. “It feels like, ‘What’s the point? It’s not going to amount to anything.’ But when you understand compounding, you realize that something that happens habitually can add up.”

Calculate the impact of small changes

Someone with a $50,000 salary who saves 1% a year will have almost $19,000 in 20 years and more than $77,000 in 40 years, assuming a 6% annual return.

Seemingly small reductions in investment fees can also produce big savings over time. According to Vanguard Group, $100,000 invested at 6% a year would grow to $429,000 after 25 years with no fees. With a 1% annual fee, the balance would grow to $339,000.

Take an experimental approach

When looking for ways to save for retirement, Mr. Sethi cautions against cutting back on things you enjoy, big or small. If you abstain from vacations, restaurant meals or a daily Starbucks habit, you might feel deprived and lose the motivation to stick with it.

To generate savings, he recommends starting with windfalls that require little effort.

Examples include getting rid of subscriptions you don’t use, negotiating discounts with cellphone or cable companies, and saving a portion of a tax refund or raise.

He recommends eventually attempting bigger windfalls, such as refinancing a mortgage or negotiating a rent reduction or salary increase and regularly reviewing your spending to identify potential savings.

Don’t get upset by setbacks, Dr. Fogg said. “Think of it as an experiment,” he said. “Figure out why it didn’t work, make changes to your approach, and try again.”

Just take one step

In contrast to eating healthier or exercising more, retirement savings can be put on autopilot, via payroll deductions to a 401(k) or automated transfers from a savings account to an individual retirement account or emergency fund.

Many 401(k) plans allow workers to agree now to automatically raise their contribution rate in the future, often by 1 percentage point a year to coincide with annual raises.

When it comes to tasks that can’t be automated, such as opening an IRA or reviewing your spending, Dr. Fogg recommends two techniques designed to make them feel easier.

The first is to take just one step. For example, if your goal is to open an IRA, simply set up a logon at a brokerage firm and program a calendar reminder to take the next step later.

Alternatively, set a timer for five minutes and tell yourself you can stop when it goes off. “When you get into it, you may find that it’s not so terrible” or that momentum propels you to finish, Dr. Fogg said.

Celebrate

When you’re done with a task, celebrate as soon as possible, if only by silently congratulating yourself.

Celebration can cause your brain to release feel-good neurotransmitters that have been found to help ingrain habits, said Dr. Fogg.

Mr. Egan said that after he and his wife conduct a monthly spending review, they treat themselves to a nice dinner and a bottle of wine.