I know you. You work hard for your money, and you're saving actively for retirement. You want to make sure you're doing the right stuff to maximize your money's effectiveness. Well, the first step to figuring out what to do is to make sure you aren't making any major mistakes. Here are three you'll especially want to avoid, because they could easily cost you $100,000 -- or more.

Missing out on the tax benefits

Uncle Sam wants you to save for retirement. If you need proof, look at the tax code -- it's littered with tax breaks designed to help savers. While retirement accounts differ in their precise design, they all share one universal benefit: They provide you with substantial tax breaks if you contribute money to them.

Most retirement accounts -- which include employer-sponsored 401(k)s and individual retirement accounts, or IRAs -- come in one of two flavors: traditional and Roth. The major difference is tax related: Traditional accounts offer you a tax break now and for the entire time your money is in the account, but you're taxed on the income after you withdraw it in retirement. Roth accounts, by contrast, give you no tax break today, but the money can grow and be withdrawn in retirement without any tax penalty ever again. Here's a good, in-depth primer if you want to learn more about them.

Just to give you a sense of the difference that taxes can make, consider two different investment accounts with $100,000 in them. Let's say these accounts will grow by 7% a year in the stock market with no further cash infusions. One of those accounts will be taxed at the capital gains tax rate each year -- which is 15% -- and will reduce gains from 7% to 5.95%. The other account -- the retirement account -- gets to skip all taxes.

Thirty years down the road, the regular investment account has done well, growing to $566,276.88. The retirement account, however, has grown to $761,225.50. That's a difference of almost $195,000. Save on those taxes.

An unhappy retiree.

Don't miss out on that cash. Image source: Getty Images.

Not maximizing the employer match

Chances are good you work for a company that offers a retirement plan -- most commonly, a 401(k). In fact, about 79% of American adults have access to a 401(k)-style plan through their employers.

An estimated 90% of employer-sponsored plans offer  a company "match" for employees who save. Usually, this is structured something along the lines of the employer putting in $0.50 for every dollar an employee contributes, up to, say, 6% of their pay. In that scenario, the employee's 401(k) would receive funds equal to 9% of the employee's salary.

That's money your company is giving you if you save for retirement. Unfortunately, roughly 25% of Americans with access to a 401(k) aren't maximizing their employer matches. On average, those workers are leaving $1,336 in matching funds on the table annually. That's free money, and it stacks up to quite a bit: Missing out on $1,336 in matching funds each year for 30 years, assuming a 7% annual return, means you'd lose over $136,000 in retirement savings.

It's free money if you just save enough to get it all. Do yourself a favor and make that happen.

Picking high-fee investments

Fees can kill investment returns, pure and simple. This is particularly true with high-fee actively managed mutual funds, which give you the double-whammy of costing a bunch in fees and underperforming the market. What a rip-off.

Fortunately, there are plenty of alternatives. Low-fee exchange-traded funds (ETFs) and index funds give you broad diversification, as they're designed to passively follow an index (like the S&P 500, the Dividend Aristocrats, or the Russell 2000). That enables approximately market-matching returns -- again, better than underperforming -- for very low fees. The average passive stock index fund charges an annual fee of 0.09% of assets under management, according to the Investment Company Institute, as opposed to the 0.82% that the average actively managed stock mutual fund charges.

Assuming, as we have above, that the $100,000 account achieves 7% annual returns -- the difference in fees over 30 years is worth a little over $136,000. That's a lot of additional cash you can gain -- without having to actually save any extra! -- simply by focusing on lower-cost funds offered in your company 401(k) plan, or by picking index funds in your IRA. For what it's worth, Vanguard funds are well respected and very low cost -- you could do a lot worse than starting there.

Save better

You work hard for the money you save for retirement. Make sure that money is doing you the most good it can by avoiding these costly retirement mistakes. A few small tweaks today -- whether it's increasing your 401(k) savings rate by a percentage point to take full advantage of the company match, opening up an IRA for your savings, or reviewing your funds to make sure you aren't getting eaten by fees -- can make a huge difference over a long time horizon.