Tuesday may be Christmas, but for most taxpayers there are still eight shopping days left; eight days to shop around for last-minute maneuvers that could cut down on 1990 tax payments and possibly 1991.

Unfortunately, shopping for 11th-hour tax breaks in the 1990 tax code is like shopping for beef in Moscow: There isn't much to choose from. And figuring out exactly how to shift income and deductions to take best advantage of the different rates has become so complicated that most accountants say it isn't worth the trouble unless some very large amount, such as a big capital gain, is involved.

Legislative changes since the 1986 Tax Reform Act have "sort of tied your hands on these timing things," said David M. Bradt Jr. of Arthur Andersen & Co. in Washington.

Jack Porter of BDO Seidman, said that when clients ask about last-minute shifting, "Generally, we tell them that unless {the amount involved} exceeds $300,000, it will cost more to tinker with the timing of the income than they will get in tax benefits."

"You have to do an incredible amount of number crunching," he added.

If it weren't already complicated, the changes enacted by Congress this fall make it even more so. Under current law, the top nominal bracket is 28 percent but certain upper-middle-income taxpayers pay a 33 percent marginal rate. Next year, the top nominal rate will be 31 percent, but capital gains are capped at 28 percent; at the same time, the value of itemized deductions will be reduced for upper-middle and upper-income taxpayers.

So a taxpayer might face a choice of higher marginal rates and better deductions this year, or lower rates and restricted deductions next year.

The calculations are mind-boggling and the results not terribly significant.

Carl W. Duyck and Barbara L. Tymec of Price Waterhouse here in the District set up two hypothetical taxpayers, one in the 28 percent bracket this year and one in the 33 percent bracket, and tried shifting $6,000 worth of income and deductions around to get the best results.

The most they could get in tax savings was less than $300, Tymec said.

"Unless you are talking about a huge transaction, it doesn't make sense to get excited" about year-end strategies, Duyck said.

There are, however, a few cases where it could pay to be alert, most accountants agreed.

One is the alternative minimum tax, which rises from 21 percent this year to 24 percent in 1991.

The AMT, as it is known, is designed to prevent taxpayers from parlaying different benefits so as to drastically reduce or wipe out their tax liability. Taxpayers with "preference items" must calculate their regular tax, then add back the preference item and other adjustments and calculate their alternative minimum tax. If the AMT is higher than their regular tax they must pay the AMT.

Taxpayers who are very close to having to pay the AMT should take special care on when to take state tax deductions. Since these are easily adjusted -- paying in either December or January -- and since these cannot be used to reduce AMT income, taxpayers in this situation should take great care in deciding which year they want the deduction in.

"You could really shoot yourself in the foot" if you are not careful, Duyck said.

Taxpayers who have nonrecurring deductions, such as occasional large charitable gifts, should consider bunching them, noted Dan Mendelson of Deloitte & Touche's Washington office. Upper-income taxpayers face a 3 percent "haircut" on most itemized deduction beginning next year, he noted. In addition, some items, such as business deductions, are subject to a threshold, which you might be able to meet by bunching them.

The historic practice of simply accelerating deductions willy-nilly "doesn't work," Mendelson noted. For one thing, the tax savings derived from prepaying items may not be enough to offset the value of holding onto that money for another year.

Taxpayers in the 33 percent marginal bracket may be able to save some money by taking a capital gain this year instead of next, but Mendelson and others said that it may make more sense to wait in the hope that market conditions will improve.

In addition, several experts pointed out that there is a one-year window for 1991 during which charitable donations of appreciated property are not treated as preference items subject to the AMT. So if you are contemplating donating to your old school the $100,000 painting you acquired in Paris in 1934 for $100, this may be your chance.

Finally, of course, there is the new excise tax on luxury cars, boats, airplanes, furs and jewelry. That kicks in at the end of the year and will cost buyers of these luxury items 10 percent of the purchase price over a base amount. The base amount on cars, for example, is $30,000, so if you buy a $40,000 roadster, you'd pay an extra $1,000 (10 percent of the $10,000 over the base price).

So if you have $40,000 or $50,000 that's burning a hole in your pocket, there are plenty of auto dealers out there who are more than ready to help you with your tax planning.