Here's an example. Say you decide to invest $10,000 all at once in one fund while your friend, who also happens to have $10,000 to invest, puts $2,000 per month in the same fund over the next five months. The fund consistently rises in value during that time. The chart below illustrates what would happen to the two investments.

 
Fund value increases
Month Your investment Your cousin's investment
1 5,556 shares at $1.80 each 1,111 shares at $1.80 each
2 * 1,099 shares at $1.82 each
3 * 1,081 shares at $1.85 each
4 * 1,070 shares at $1.87 each
5 * 1,053 shares at $1.90 each
Total shares 5,556 5,414
Ending value $10,556 $10,287
* Not available

You would end up ahead, because you own more shares at the end of the five-month period. And you own more shares because, due to the consistently rising value of the fund, your friend couldn't afford to purchase as many shares as you had purchased originally. But what happens if the value of your fund fluctuates dramatically during those five months?

 
Fund value fluctuates
Month Your investment Your cousin's investment
1 5,556 shares at $1.80 each 1,111 shares at $1.80 each
2 * 1,667 shares at $1.20 each
3 * 1,081 shares at $1.85 each
4 * 1,481 shares at $1.35 each
5 * 1,053 shares at $1.90 each
Total shares 5,556 6,393
Ending value $10,556 $12,147
* Not available

In this case, your friend ends up in the lead. By investing a fixed dollar amount in the fund every month, your friend bought more shares when the price was low, fewer shares when the price was high and ended up with more shares after five months.

To be sure, such drastic fluctuations in net asset value are rare. Because the stock market generally goes up more often than it goes down, most investors will receive the best long-term results by lump-sum investing.

Why dollar-cost average?

Investing in dribs and drabs may not be the path to greater return, but we still think dollar-cost averaging, or investing a set amount on a regular basis, is a great method of investing. Incidentally, if you contribute to a 401k plan at work, you're already investing this way. Our argument for dollar-cost averaging has a couple of dimensions. First, dollar-cost averaging can reduce risk. If your mutual fund declines in value, the worth of your investment is less, even though you still own the same number of shares. In the same way that dollar-cost averaging will net you more shares in a declining market, it can curtail your losses as the fund goes down. The chart below illustrates this point.

 
Fund value decreases
Month Your investment Your cousin's investment
1 5,556 shares at $1.80 each 1,111 shares at $1.80 each
2 * 1,250 shares at $1.60 each
3 * 1,379 shares at $1.45 each
4 * 1,538 shares at $1.30 each
5 * 1,667 shares at $1.20 each
Total shares 5,556 6,945
Ending value $10,556 $8,334
* Not available

In this example, both you and your friend lost money (remember, you each started with $10,000), but your friend lost less by dollar-cost averaging. She had cash sitting on the sidelines that did not lose value. And when the fund rebounds, your friend also will be in better shape because she owns more shares of the fund than you do.

The second reason we like dollar-cost averaging is that it instills discipline. Investors often chase past returns, buying funds after a hot performance streak. And they'll sell funds when returns slow or decline. Bad idea: That's a form of market timing. But dollar-cost averaging prevents you from market timing, because you're buying all the time. Heck, you may even forget that you're investing if you set up an automatic-investment plan with a mutual fund family.

Which leads us to the final reason we love dollar-cost averaging: It's a crafty way to invest in some great mutual funds that might be inaccessible otherwise. Many fund companies will waive their minimum initial investment requirement if you agree to set up an automatic-investment plan and invest a little each month or quarter.

Decision time

While market timing is out of the question for all investors (but some still try), whether you invest all at once or a little at a time depends on how much time you have to invest and whether your primary goal is maximizing return or minimizing risk.

The shorter your time horizon, the greater chance you take of losing money with a lump-sum investment. However, if you had $20,000 to invest, it probably wouldn't make much sense to invest $1,000 per year for the next 20 years. Over long time frames, funds go up more often than they go down, and when they go down, they eventually bounce back. It is almost certain that the NAV you would pay 10 years from now would be higher than the NAV you would pay today.

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We suggest combining the two strategies: Invest as much as you can today and vow to invest a little more each month or quarter. That'll keep you disciplined and have you investing right away.