One of the simplest ways to boost your retirement savings is to lower your investment costs. Research has repeatedly shown that funds with high expense ratios tend to under-perform their cheaper siblings.

That's because high fees are no guarantee of superior performance. In fact, they take a bite out of performance.

So all things being equal, a lower-cost fund is more likely to give you a higher return without your taking any additional risk.

For example, if you invest $100,000 in a fund with a 1.5% expense ratio (the average for U.S. stock funds) and the fund gains an annualized 8% over 15 years, you will end up with $317,000.

If that same fund had an expense ratio of just 1%, you would earn an 8.5 percent return, which would produce $340,000, or $23,000 more.

If you currently own funds with high expense ratios, consider swapping into low-cost substitutes. It's easy to make an exchange in a 401(k) or an IRA, since there are no tax consequences.

In your taxable accounts, you'll need to weigh the possible tax bill before cashing out, but you can at least begin directing new money into lower-cost funds. Again, the funds on our Money 70 list all carry below-average expense ratios.