People work hard their whole lives to relax and enjoy life in retirement. But financial mistakes can turn your golden years into a financial struggle. Many retirees make the same money mistakes over and over -- sometimes without even realizing it. Here are five common retirement mistakes, and how to avoid them. 1. Underestimating how long retirement will last With medical advancements and healthier lifestyles, many retirees today are living well into their 80s and even 90s. That means a retirement fund may need to last 25 to 30 years or more. Unfortunately, many retirees under-save or withdraw too much too soon. How to avoid it: Use the 4% rule -- withdraw no more than 4% of your savings per year.Consider delaying Social Security to maximize lifetime benefits.Keep some money invested in stocks to hedge against inflation. 2. Claiming Social Security too early Many retirees are eager to start collecting Social Security as soon as they become eligible at age 62. However, claiming benefits early comes with a major downside: permanently reduced payments. Those who claim Social Security at 62 could see their monthly benefits cut by as much as 30% compared to waiting until full retirement age (66 or 67, depending on birth year). Delaying Social Security benefits can lead to much higher lifetime earnings, as benefits grow by about 8% per year until age 70. If other sources of income are available, it often makes sense to wait. 3. Ignoring inflation's impact on savings Inflation is a silent threat that can erode purchasing power, yet many retirees don't plan for it. Even at a modest inflation rate of 3%, the cost of living could double over a 25-year retirement. What seems like a comfortable amount of savings at the beginning of retirement may not stretch nearly as far 20 years down the line. One of the best ways to combat inflation is to keep a portion of investments in the stock market, which has historically provided higher returns than bonds or cash. But when it comes to where to keep your cash, retirees should also consider having a high-yield savings account (HYSA). HYSAs can earn more than 10 times the national average interest rate of 0.41%. Check out our list of the best high-yield savings accounts now. 4. Keeping too much money in cash It's tempting to move all of your money into safe assets like cash or CDs, but doing so can actually be risky. If your money isn't growing, inflation will shrink your wealth over time. How to avoid it: Keep some money invested in diversified stocks and bonds.Use a high-yield savings account for emergency funds instead of a regular savings account. 5. Forgetting about taxes on retirement income Just because you're retired doesn't mean taxes disappear. Many retirees don't realize that withdrawals from 401(k)s, IRAs, and even Social Security could be taxed. How to avoid it: Consider Roth account conversions to reduce taxable income later.Withdraw from tax-advantaged accounts strategically.Work with a financial advisor to create a tax-efficient withdrawal plan. Make the most of your money If you're approaching retirement or already there, now is the time to evaluate your financial plan and make adjustments where needed. Create a checklist to ensure you're not making mistakes that could be costing you dearly.