It’s no good putting this off any longer: time to start thinking about next April’s tax return.

So soon?

Yes — assuming you want to keep your 2003 taxes as low as possible. The most valuable tax maneuvers have to be complete by the end of this calendar year, and they take some time to enact.

Also, the federal tax cuts passed in May cause some changes in the annual year-end tax-planning ritual.

The basic goal, though, remains the same — to reduce taxes, or to postpone them for another day. You eventually will have to pay a tax on a profitable investment, but postponing it leaves that tax-payment money compounding in your accounts longer.

The main exception is the person who expects to be in a higher tax bracket in the future. In that case, it’s better to pay tax at today’s lower rates.

Taxes are most easily postponed by delaying the receipt of income. If you are owed money, for example, you can hold off sending the bill so you won’t receive payment until after Dec. 31. Or you can ask your boss not to pay your year-end bonus until 2004.

Investors often have opportunities to reduce taxes by selling money-losing investments. Even though stocks have rebounded this year, many stocks and mutual funds are still trading well below their peak prices of several years ago, meaning they contain losses that can be “realized” by selling shares for less than was paid when they were bought.

Realized losses can be used to offset profits earned on other investments that were sold or will be sold in 2003. If realized losses are bigger than realized gains, the losses can be used to reduce your ordinary taxable income by as much as $3,000 a year, thus reducing your income tax. Any additional losses can be saved, or “carried forward,” for offsetting capital gains or reducing taxable income in future years.

Suppose you had recently sold a block of mutual fund shares that had made $10,000. If you had owned them for longer than 12 months, the profit would be subject to the 15 percent tax on long-term capital gains. The tax would be $1,500.

But if you sold another long-term holding that had lost $10,000, your net gain would be zero, and you’d save $1,500 in taxes.

If you sold an investment that had lost $15,000, you’d not only eliminate that $1,500 tax, but you could save the tax on $3,000 in ordinary income, and have an additional $2,000 in losses to offset capital gains or income next year.

To get the most out of tax-loss sales, it’s important to consider the different rates.

The tax cuts passed last spring reduced the long-term capital gains tax, paid on investments owned more than 12 months, to 15 percent from 20 percent for most taxpayers. People in the 10 percent and 15 percent income tax brackets now pay only 5 percent on long-term capital gains.

In addition, income tax brackets were reduced: They are now 10, 15, 25, 28, 33 and 35 percent. Those also are used for short-term capital gains, the tax on investments owned 12 months or less.

In selling a profitable investment, it’s best to sell one owned longer than a year, since the highest tax rate will be 15 percent rather than the 25 percent to 35 percent most investors pay on short-term capital gains.

But in selling losers, those held a year or less can be the most beneficial, since they can offset short-term gains on other investments that might be taxed at the higher rates.

As you see, there can be very big tax savings for investors who are careful in deciding what to buy and sell, and when.

But don’t buy or sell an investment just to save taxes. The investment’s prospects come first. A holding that has lost money may well be worth keeping if its future looks bright, while a winner you love is worth selling if you think its best gains are behind it.

You also can sell just part of a losing investment, keeping the rest for a possible rebound. That would still free some cash for another investment and also give you a tax loss.

If the investment had been accumulated over time, the investor can sell the shares she paid the most for, generating the biggest tax loss possible.

Your broker or fund company can tell you how to specify the shares you sell.

A smart investor can save thousands with careful year-end tax planning. But it’s tricky, so don’t wait ’til the last minute.