For many retirees, Social Security benefits are the only thing keeping them afloat.

Although your benefits are designed to replace only around 40% of your pre-retirement income, half of baby boomers say they expect their benefits to be their primary source of income, according to a survey from American Advisors Group, a leading reverse-mortgage company. In addition, roughly 1 in 5 married couples depend on their monthly checks for at least 90% of their income, the Social Security Administration (SSA) found.

While it's not ideal to rely on your benefits more than you should, if you're nearing retirement and your savings are sparse, you may not have any other choice. If so, it's crucial to maximize your monthly checks. And there are at least three ways you may be hurting your Social Security benefits without even realizing it. 

Stacked Social Security cards

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1. Not working a full 35 years

Before you can maximize your benefits, it's important to understand how they are calculated. The SSA will first take an average of your 35 highest-earning working years (called your average indexed monthly earnings, or AIME), then apply a formula to it to account for any changes in general wage levels. Your monthly benefit amount is then determined based on the result of this calculation.

The part of this calculation that you can control is the number of years you work. If you don't work a full 35 years, you'll have zeros inputted into your calculation -- which brings down your AIME. With a lower average income, you'll see a lower basic benefit.

Even if you have worked at least 35 years, you may want to continue a few years longer if you can. Because your average is based on your highest-earning years and you're likely earning a much higher salary as you near retirement than when you started your career, you can replace some of your lower-earning years with more current, higher-earning ones. The result is a higher average income and a larger benefit amount as well.

You can also see how much you can expect to receive in future Social Security benefits by creating a mySocialSecurity account to check your statements online. These estimates are based on your real earnings, giving you an accurate idea of how much you'll likely be receiving.

2. Not claiming at the right time

The basic benefit you're entitled to assumes you'll be claiming at your full retirement age (FRA), which is age 67 for those born in 1960 or later, or age 66 or 66 and a few months for those born before 1960. If you claim before or after that age, your benefits will be affected.

You can begin claiming Social Security as early as age 62, but you'll receive smaller checks. In fact, if your FRA is 67, your benefits will be reduced by 30% if you claim at 62. But if you wait until after your FRA to claim (up until age 70), you'll receive a bonus amount on top of your full checks. Those whose FRA is 67 can expect to receive an additional 24% each month by waiting until 70 to claim. While you can wait until after 70 to claim, benefits won't continue to increase -- so there's no financial incentive to do so.

Typically, these changes are permanent once you start claiming. If you change your mind after you begin claiming benefits, you have one opportunity to reverse your decision. You'll need to do so within 12 months of when you started claiming, and you'll also have to repay all the benefits you've already received.

At first glance, it may seem obvious: Hold off on claiming benefits for as long as you can to receive those bigger checks. But that's not always the case. For example, if you have health issues and don't expect to live that long, you may miss out on thousands of dollars in benefits by waiting longer to claim. On the other hand, if you expect to live a long time and don't think your personal savings will last that many years, waiting as long as you can to collect those fatter checks can result in more income every month for the rest of your life.

3. Not taking advantage of spousal benefits

If you're married, you may be able to collect benefits based on your spouse's work record. For those who are currently married, you must be at least 62 to claim based on your spouse's record, and you also can't start receiving benefits until your spouse begins claiming. When you do start claiming, you can receive up to 50% of your spouse's full benefit amount.

Even if you're divorced, you may still be able to claim based on your ex-spouse's record if you were married for at least 10 years and are not currently married (although if your spouse remarries, you may still be eligible to claim on his or her record). You must be at least 62, but in this case, you don't have to wait until your ex-spouse claims before you can begin receiving benefits as long as you've been divorced for at least two years. Also, regardless of whether you're married or divorced, if you claim benefits based on your spouse's record, your spouse's benefits will not be affected.

You can also file for spousal benefits even if you're entitled to benefits of your own. The SSA will pay out your benefits first, then if you could be receiving more based on your spouse's record, you'll receive an extra amount to make up the difference.

Finally, keep in mind that all normal claiming rules still apply here. While you can claim as early as age 62, you won't receive the full amount you're entitled to unless you wait until your FRA to claim.

Social Security benefits can be complex and confusing, but it's important to understand how they work if you want to make the most of them. Especially if you don't expect to have enough savings to last the rest of your life, maximizing your benefits can help you enjoy a more financially stable retirement.