Even ordinary taxpayers can keep more of their earnings by planning

''There is nothing sinister in so arranging one's affairs as to keep taxes as low as possible. Everybody does so, rich or poor, and all do right. . . .'' To the ''poor,'' or those of only modest means, the words of noted American jurist Learned Hand may seem a bit hollow. What, after all, can anyone but the rich do to ''arrange one's affairs to keep taxes as low as possible''?

It turns out there are several things people in moderate income brackets can do to reduce the federal tax bite. Some are simple and require nothing more than filling out a few forms at a bank, brokerage, or insurance company; others require a little more effort.

* IRAs. The easiest all-income tax shelter, a 1981 gift from Congress to anyone who earns wages or a salary, is the individual retirement account (IRA). By putting up to $2,000 a year (yes, you can put in less) into an IRA, you can reduce your taxable income by the amount deposited, whether or not you itemize deductions. If you are married and your spouse doesn't work, you can deposit up to $2,250. If you both work, two separate IRAs can hold a total of $4,000.

Until you retire, this money earns interest free of federal taxes.

IRAs are available to all, no matter what the income, so even the ''poor'' can start them with a few dollars a year to build up a savings habit, as well as the beginnings of a retirement fund.

* Keoghs. A close relative of the IRA, the Keogh is for people who are self-employed. But it can also be used by people who have a fair amount of outside, or free-lance, income. The lesser of 15 percent or $15,000 of income can be put into the Keogh; for this year, the maximum goes up to $30,000. If you haven't opened a Keogh yet, you won't be able to put any 1983 contributions into it now; they must be opened by the end of the calendar year. IRAs for 1983, however, can be opened anytime until you file your return.

* 401(k)s. Also known as salary-reduction plans, these look similar to IRAs but have some important differences. First, they must be sponsored by an employer, and the annual deposit limits are higher. Another important difference is that money deposited in a 401(k) comes out of your pay before any federal or social security taxes are taken out, thus putting you in a lower effective tax bracket throughout the year.

In addition to putting some of your own money in the 401(k), your employer almost always has a matching amount to put in. This match usually ranges from 1 to 3 or 4 percent.

* Shifting income. ''One of the most important techniques is shifting income between years and between family members,'' says Jonathan Flitter, manager of employee benefits in the Boston office of Touche Ross, the accounting firm.

As tax rates and tax brackets have declined over the past three years, Mr. Flitter explained, it has become an annual fact that if you can take any income in a later year, you'll pay less tax on it than if you take it in the earlier year. So if your boss is planning on giving you a year-end bonus, ask him to hold it until January. If you do any free-lance work and you can get the client to pay you in January instead of December, the taxes on it will be lower. And, if you want to sell some stocks, it may make more sense to sell these securities in the next year - particularly if you've had a gain - than in the current year.

As for relatives, Mr. Flitter notes, taxpayers will often do better if they can find ways to shift some of their income to relatives, such as children, who are in lower brackets. Something as conservative as a mutual fund, for example, might be taken out in the child's name. Then, any money it earns while the child is growing up would be taxed at the lower rate.

You can also shift additional income or any sudden windfalls to your children's brackets through Unified Gifts to Minors Act custodial accounts, Clifford trusts, or interest-free demand loans. A qualified tax attorney or tax lawyer should be consulted before setting these up.

There is also the gift-tax exclusion, which allows you to give $10,000 a year ($20,000 for a married couple) to children, or any recipient, free of gift tax. The children must pay tax on the gift, but again, their lower bracket eases the burden.

* Charitable contributions. Americans are a generous lot. Some 90 percent of the money that goes to private charities comes not from corporations but from individuals. For its part, the US government does what it can to encourage giving: Charitable contributions are the only deduction allowed on the simplified 1040EZ form.

So there is no reason contributions cannot be used to the taxpayer's advantage. If you think you are going to have more income this year than next, for instance (a situation that will probably be more evident later in the year), you can make your 1984 and 1985 contributions this year. Some people, in fact, alternate their contributing years, making little or no contributions in one year, and double-size donations in the other year.

If you do any volunteer work for your charity, there are specific things that can be deducted. Deductible items include expenses for transportation, food, and lodging while away from home as well as uniforms, equipment, and supplies, if these expenses were incurred for a qualified organization. If you use your own car for the volunteer work, you can deduct 9 cents a mile for its use.

* Income averaging. In recent years, many families have added a wage earner, when the wife, for example, returned to the work force. This may result in a substantial change in the family's taxable income. To help taxpayers spread the effects of such a change over several years, the IRS permits income averaging. By filling out a little over half of Schedule G, where you compare your 1983 income with the previous four years, you can find out if you are eligible for income averaging. If you are eligible, complete the form.