You'll often hear that you'll need money outside of Social Security to enjoy a comfortable lifestyle in retirement. And that's where your personal savings come in.

If you have access to a 401(k) plan through your job, it may be your go-to source for building a nest egg. But just plain contributing to a 401(k) isn't enough. You'll also need to make sure you're getting the maximum benefit from that account and investing your savings wisely. And that means avoiding these big mistakes.

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1. Leaving free matching dollars on the table

Many companies that offer 401(k) plans also match worker contributions, to some degree. If you're not contributing enough to your savings to snag that full match, then you're effectively passing up free money.

This year, a lot of people have had no choice but to cut back on 401(k) contributions, due to inflation. And to be clear, you should put your immediate needs ahead of your long-term ones when it comes to things like food and shelter. But if you've been neglecting your 401(k) due to prioritizing leisure spending and not inflation, then it's important to get yourself back on track -- and start contributing enough to claim your employer match in full.

2. Not choosing your own investments

If you don't choose a specific set of funds for your 401(k) dollars, you'll generally land in your plan's default investment option. That's usually a target-date fund -- and may not be your best bet.

Target-date funds are designed to shift investments around based on risk as milestones near. They allow savers to take a hands-off approach to investing, which might seem like a good thing when it may not be.

The reality is that target-date funds have their share of flaws. Many charge expensive fees, and some might invest your money too conservatively, thereby limiting your nest-egg's growth. If your 401(k) dollars are stuck in a target-date fund, you may want to look at other options as soon as possible.

3. Choosing investments that are really expensive

Maybe you did actively invest your 401(k) dollars, only you opted to put your money into actively managed mutual funds. That's not necessarily a poor choice if those funds are delivering solid returns. But if that's not the case, then it may be time to pull your money out and move it into index funds instead.

Index funds are passively managed funds with fees that can be significantly lower than what actively managed mutual funds charge. And often, you'll find that the performance of index funds is comparable to that of mutual funds -- in which case, there's no sense in paying a premium if you're not actually getting a better return.

Your 401(k) might end up being a major source of retirement income. So it's important to avoid mistakes that could hinder your savings efforts -- and leave you with less money than you'd like as a senior. If you've fallen victim to these blunders, do your best to address them without delay.