As you're preparing for retirement, it's easy to get caught up with the big questions: How much should I save? What age should I retire? When should I claim Social Security benefits?

While these are all important questions, it's equally critical that you're thinking about the more subtle aspects of retirement planning as well. These three mistakes are easy to overlook, but they could cause serious problems down the road.

Older couple sitting on a couch looking at documents and a calculator

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1. Waiting too long to begin saving

Your savings rely on compound interest to grow. Compound interest means you're earning interest not just on your direct contributions but also on all the returns you earn on your contributions.

The longer you wait to begin saving, the less time compound interest has to do its job. As a result, you'll need to do more of the legwork by contributing more money to your retirement fund each month.

Say, for instance, you started saving at age 25 and had a goal of saving $750,000 by age 65. If you were earning average stock market returns, you'd need to invest just over $300 per month to achieve that target. But if you had waited until age 40 to start saving, all other factors remaining the same, you'd need to save around $1,000 per month.

If you're already behind on your savings, though, don't give up hope. It's never too late to start, and starting now is far better than saving nothing at all.

2. Withdrawing money from your retirement fund

Your retirement fund may look like an enormous savings account, making it tempting to tap into your investments if you're tight on cash. And in some cases, you may have no other option but to do so. However, withdrawing from your retirement fund should be a last resort, because it could be riskier than you think.

Generally, when you withdraw from your 401(k) or traditional IRA before age 59 1/2, you're subject to a 10% penalty and income taxes on the amount you withdraw. In addition, every time you take money from your retirement fund, it becomes more difficult for your savings to grow.

This, again, goes back to your compounding returns. When you withdraw from your retirement fund, that lowers your account balance. The lower your account balance, the less you'll earn on those savings. And sometimes, taking even a relatively small withdrawal from your retirement fund could mean missing out on hundreds or even thousands of dollars in potential growth down the road.

If you've already withdrawn from your retirement fund, it's not the end of the world. Focus on building an emergency fund so that the next time you're hit with an unexpected expense, you won't be forced to raid your retirement savings.

3. Investing too conservatively

Investing in the stock market is inherently risky, so you may choose to put your money in more conservative investments like bonds to limit your risk. While that's a good idea in theory, it could significantly limit your earning potential.

Bonds and other conservative investments may generally be safer than stocks, but they also have much lower average rates of returns than stocks as well. When you're not earning as much on your investments, that will make it more challenging to reach your savings goals. 

In addition, stocks may not be as risky as they seem. While stocks do experience more volatility in the short term than bonds, over time, they tend to see positive returns.

^SPX Chart

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The one caveat to consider here is that when you're approaching retirement age, you should take a more cautious approach. Investing aggressively is a great option when you have decades left to save. But if you're going to need your money within the next few years, you should play it safe.

Preparing for retirement is tough, and everyone will make mistakes sooner or later. By learning from these mistakes, though, you can make better financial decisions and save more for retirement.