Money gurus are always preaching long-term investing. Not only will that give you a better shot at earning more, it'll also get you a lower tax rate when you sell.

But exactly what capital gains tax rate you pay depends on several things, including when you bought the asset, when you sold it, your overall income level, and sometimes, what tax-code changes are made in the meantime.

Currently, capital gains are at historic lows. Some taxpayers in the two lowest tax brackets could end up without any capital gains tax bill. That's right, zero capital gains for some filers.

Most investors will see their gains taxed at 15 percent. And 25 percent and 28 percent rates apply in special circumstances.

One thing all these tax levels do have in common is that they are known as long-term capital gains. This means they apply to assets that you hold for at least 366 days (more than one year).

Zero capital gains taxes for some
On Jan. 1, 2008, the best of all possible tax rates -- zero percent -- took effect for investors in the 10 percent and 15 percent income tax brackets.

Previously these taxpayers had to pay Uncle Sam 5 percent of their long-term capital gains. Now any long-term assets they sell will be exempt from capital gains taxes.

To qualify for the zero rate, a married couple could make in 2008 no more than $65,100 in taxable income; single filers earning $32,550 or less will pay no tax on the sales of assets they've owned for more than a year.

While lower-income individuals aren't typical investors, this tax benefit could help out folks such as retirees who have little or no taxable income. And the children of older individuals could combine the annual gift exclusion ($12,000 in 2008) with this capital gains break and give appreciated long-term assets to their older parents.

15 percent tax rate for most
The zero-percent rate is just the latest in a series of investor-friendly tax changes enacted during the George W. Bush administration. Prior to his taking office, investors whose overall income put them in the top four income-tax brackets faced a long-term capital gains rate of 20 percent, while lower-income investors paid capital gains taxes of 10 percent.

Tax-law changes in May 2003, however, lowered the rates by 5 percent each, with the lower rate, as noted earlier, eventually being zeroed out in 2008.

The changes have had the most effect on investors in the higher income ranges, 25 percent to 35 percent tax brackets. These individuals now find their capital gains taxed at 15 percent. This lower rate also applies to some dividends that stocks and mutual funds pay account holders. When you hear "lower capital gains rate," it generally means this 15 percent level, because there are few investors with incomes low enough to qualify solely for the 5 percent, now zero percent, rate.