You're setting aside 15% of your income for retirement and using tax-advantaged accounts like a 401(k) or IRA. So far, so good. But unless you're directing those funds to the right asset classes, you could end up with less-than-optimal investment results.

Choosing the right investment mix is one of the absolute pillars of long-term financial planning. On the one hand, if you get too aggressive with your purchases, you'll become vulnerable to the inevitable market downturns. On the other, if you become too cautious, your portfolio may not grow enough to support a lengthy retirement.

The key is striking the right balance, given your age and financial goals. Here's a look at how to find that sweet spot.

While a retirement portfolio can contain any number of different investment vehicles, most financial planners suggest making stocks and bonds the core of your portfolio.

One advantage of a stock/bond mix is that the two assets complement each other. Stocks offer more growth potential over the long haul. But they're more volatile, especially with time horizons shorter than 10 years.

Historically, bonds offer slightly lower gains than equities, but they also have fewer ups and downs. So you can buy them with greater confidence that they'll hold their value over a shorter time frame – say, 3 to 10 years.

Figure 1. Historically, stocks provide significantly higher returns than bonds, but are more volatile. Steering part of your portfolio toward fixed-income assets helps mitigate the risk of a market downturn.

: Russell Investments

As you edge closer to retirement, you may want to set aside a portion of your nest egg for even more immediate needs. Some experts recommend putting one or two years' worth of expenses in cash and cash equivalents, like short-term CDs and money market accounts.

This way, you end up with three different investment buckets. Your first bucket, comprising mostly stocks, is the engine that can help grow your account over a decade or more. Your second bucket, loaded with short- and intermediate-maturity bonds, provides modest growth and safeguards against inflation. The last one consists of ultra-low-risk assets for principal protection, even in the midst of a downturn.

Providing for your short-, medium- and long-term needs probably sounds sensible enough. But the real question remains: How much should you put in each bucket?

Generally speaking, it's a good idea to lean more heavily on stocks when you're young and gradually increase your bond allocation as you get older. For years, the widely cited rule of thumb was to designate a percentage of equities equal to 100 minus your age.

But with Americans living longer, that well-worn axiom is losing favor. A quarter of people who reach age 65 will live to at least the age of 90, so it makes sense to plan for 30 or more years of needing income after you leave the workforce. Therefore, it's becoming more common to hear financial planners talk about a 110 minus your age or even a 120 minus your age rule in order to drive portfolio growth early on in retirement.

For a 70-year-old with a modest risk tolerance, that means keeping at least 40% of your assets in the stock market, with most of the remainder earmarked for bonds. By that time, it doesn't hurt to also have a small carve-out for cash and money market accounts to cover your short-term needs.

Figure 2. The following are typical model portfolios based on investor age. A moderate-risk portfolio is generally appropriate for investors in their 60s, while those in their 70s tend to adopt a moderately conservative mix. A more conservative allocation, based largely on bonds and cash instruments, is more suitable for retirees over the age of 80.

: The Charles Schwab Corporation

It's not just a matter of finding the right split between stocks and bonds – it's also important to diversify within these categories. Shares of bigger corporations sometimes perform better or worse than small-cap equities. Having a portfolio with a good mix of both can help stabilize your returns.

You may also want a chunk of your stock allocation reserved for international stocks, especially those that invest in developing markets with a strong growth potential. More aggressive investors, including those early in their career, may want foreign equities to comprise somewhere between 20% and 30% of their total portfolio. As you get older, you can slowly back off in favor of lower-risk investment options.

On the fixed-income side, consider a mix of funds that focus on short- and intermediate-maturity bonds. That way, you'll have a nice hedge against interest rate fluctuations. Some investors also like the peace of mind that comes with inflation-protected bonds, which rise in value when the cost of living increases.

So far we've discussed the bread and butter asset categories – stocks, bonds and cash equivalents. But what about alternative investments, like real estate and gold?

The former, in particular, can be a sound way to further diversify your savings. One of the more common ways to participate in the real estate market is through real estate investment trusts (REITs), which invest in land, commercial developments and loans.

Over a 25-year period the total return on REIT stocks has performed about as well as the broader stock market. But even more important, they've proven a good complement to stocks because the two don't always move in lock step with each other. In part, that's because the trusts generate income from real estate leases, which provide a relatively stable of cash.

In other words, they add balance to your portfolio without requiring you to sacrifice returns. So moving a few of your stocks over to these trusts can be a winning formula. See 3 REITs That Are Top Bets for Retirement in 2016.

Precious metals are another matter. The primary argument for investing in gold is that it acts as a hedge against inflation. When the value of the dollar is declining, why not turn to a finite re with inherent value, right? The problem is that, when you actually look at the numbers, the correlation between gold values and overall consumer prices is really pretty murky.

Even more troubling is gold's tremendous volatility, even compared to the stock market. The shiny metal had a nice run leading up to its most recent peak in 2011. But since then, the spot price has plunged roughly 40%.

The simple fact is that there are better ways to protect your portfolio against inflation – like REITs and inflation-protected bonds – that won't put you on a dangerous roller-coaster ride.

An asset mix that reflects your age and investment objectives is one of the keys to a low-stress retirement. While it's appropriate to become a little more conservative as you get older, you don't want to become so cautious that you outlive your assets.

For more, see Should I Invest in Bonds During My Retirement?, Should I Invest in Stocks During My Retirement? and Midlife Retirement Planning Guide.