Ready for a scary sound bite? It comes straight from the Social Security Administration: "Among elderly Social Security beneficiaries, 21% of married couples and about 45% of unmarried persons rely on Social Security for 90% or more of their income."

Those numbers should inspire at least a shudder, given that Social Security benefits generally only replace about 40% of your pre-retirement income. Think about it: If you suddenly had to live on 40% of your salary, that lifestyle would feel pretty dismal.

You can avoid that fate by following a long-term savings plan using your workplace 401(k). But a viable retirement plan goes well beyond setting up your 401(k) contributions. Here are three major mistakes that'll work against you, even if you're diligently contributing to your employer-sponsored retirement plan:

Woman looking stressed out while looking at a laptop.

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1. Taking money out of your 401(k)

According to a study by Fidelity, one-third of 401(k) account holders cash out their balances after a job termination. The same study also concludes that 20.2% of 401(k) participants have taken out loans against their retirement savings.

Cashing out or borrowing money from your 401(k) can have disastrous, long-term consequences. You aren't just lowering your account balance, you're also lowering your future earnings potential. If you cash out $5,000 from your 401(k), you miss out on about $2,090 in earnings over the next five years, assuming a 7% average annual return. In 10 years, you've forgone $5,050 in earnings. Those lost earning opportunities are tough to recoup, even if you're disciplined about replacing the funds through higher contributions.

The only solution is to consider your 401(k) funds off-limits. If you change jobs, do a direct rollover to a new plan or an IRA. In a direct rollover, the funds are transferred to the new account electronically. An indirect rollover, however, leaves you with a check in your hand -- and that's just too easy to cash. Also, avoid borrowing from your 401(k) unless it's truly your last option.

2. Using debt to pay for emergencies

Debt in general is not a good way to cover surprise expenses. You'll end up rolling over those debt balances, and the interest expense will take a chunk out of your household budget every month. You'll have to cut back in other spending areas to offset those interest charges. And if you don't, you'll feel even more pressure on your cash flow. The big risk you take is falling into a debt spiral where you have to keep borrowing to make ends meet.

The remedy is to start saving in an emergency fund today. Your goal is to have enough cash on hand to cover three to six months of living expenses. That way, you can dip into those funds if you lose your job, your car breaks down, or something else crazy happens. Try setting aside 5% of your income in a high-rate savings account as a starting point. Then, tap those funds first when emergencies come your way. Replenish your balance whenever it falls below the equivalent of three months of living expenses.

3. Spending more than you make

You already know that routinely spending more than you make puts enormous pressure on your household finances, but it's still so easy to do, and sometimes without realizing it.

In this era of digital transactions, you may not pay much attention to the ebb and flow of your bank account balances -- until you are accidentally overdrawn or you don't have enough in your checking account to pay off a credit card bill. Or you may think you have your spending in check until you get one of those big annual bills, and you don't have the cash on hand to pay it.

If you are outspending your income today, you're basically "pre-spending" your next raise. You'll see your cash savings decline and your credit card balances increase. Naturally, you'll look to your next raise to offset those trends. The challenge here is that you end up taking away one of the most powerful means of growing your retirement savings: increasing your contributions every time you get a raise. If the raise has to catch you up from overspending in the past, you won't feel like you can afford to increase your contributions. And without those increases, you'll struggle to accumulate a high balance in that 401(k).

The only way to break the cycle is to create a budget and stick to it. Start by adding up your required expenses and comparing them to your net income. If your income falls short before your discretionary spending, you'll need to make some big changes. Consider moving to a cheaper place or selling your car in favor of public transportation. Look for subscriptions and services to cut back or cancel, like cable, streaming TV, internet, cellphone service, and gym memberships.

Rein in those expenses until your net income covers required expenses, retirement contributions, and emergency fund savings, while still leaving some room for fun stuff like entertainment and clothes. If you don't allocate anything for entertainment, you'll find your budget very hard to follow.

Manage spending today to save

Long term, your goal is to save up as much as you possibly can in your retirement accounts, while remaining debt-free. That requires disciplined spending and saving on your part, but the rewards are huge. You won't be fully dependent on Social Security, which means you'll be far better positioned to maintain your lifestyle after you leave the workforce.