Contributing to a 401(k) for many years could be your ticket to a financially stable retirement. But simply putting money into a 401(k) plan year after year isn't enough.

It's important to invest your 401(k) mindfully and be strategic about funding and managing that account. And part of that means avoiding these key mistakes that could cause you serious problems once retirement arrives.

Image source: Getty Images.

1. Not contributing enough to get your full employer match

Many companies offer to match worker contributions to their 401(k)s. That could mean your employer will match a percentage of your salary or a preset dollar amount -- for example, give you up to $3,000 a year toward retirement savings regardless of what your salary is.

A workplace match is basically free money for your retirement. And if you don't contribute enough to your 401(k) to claim your match in full, you could end up with a lot less money once you're ready to start tapping your savings.

One thing to remember about 401(k) matching dollars is that you're able to invest that money. So over time, it could grow into a large sum.

If your company offers a generous match, consider pushing yourself to work a side hustle so you're able to capitalize on it. You never know when you might switch jobs and end up working someplace where there's a much smaller 401(k) match, or no match at all.

2. Cashing out your savings too early

Speaking of switching jobs, if you move from one company to another and cash out your 401(k) balance each time you do, you could end up with a serious problem once retirement rolls around.

First of all, if you cash out a 401(k) before turning 59 and 1/2, you typically face a 10% early withdrawal penalty on your balance. So even though you might think it makes sense to pocket a 401(k) with $2,000 in it rather than deal with the hassle of rolling it into another retirement account, doing so will mean handing $200 over to the IRS right off the bat.

Also, if you keep cashing out your 401(k) balances when you move jobs, you risk an eventual shortfall. So rather than do that, have a plan when you swap employers. You can always roll a 401(k) into an IRA if your new company doesn't have its own retirement plan.

3. Choosing the wrong 401(k) investments (or not choosing at all)

The investments you choose for your 401(k) could spell the difference between meeting your savings goals in time for retirement or falling short. And one thing you don't want to do is invest your money too conservatively.

But some 401(k) savers do that without even realizing it. And if your 401(k) is set up to have your money automatically land in a target date fund, you could end up with a slow-growing portfolio.

Target-date funds are designed to adjust your asset allocation and risk profile based on your anticipated retirement date. While they're an easy way to invest, they could stunt your savings' growth.

Some 401(k)s default to target date funds when participants don't choose their own investments. So it's important to review your 401(k)'s fund choices and pick the ones that lend to growth. Broad-market index funds are often a good option, and their passive management tends to mean low fees.

The purpose of having a 401(k) is to set yourself up for a secure retirement. So to that end, make sure to claim all the free money you can get, leave your money alone when you switch jobs, and be mindful of the way your 401(k) is invested.