The clampdown on consumer credit couldn't have hit taxpayers at a worse time. People who planned to cover their tax bills with overdraft checking or a personal loan may now be told by their bankers, "no dice."

Not all income-tax loans are out of the question. Depending on how much you need, your level of debt and your relationship with the bank, you may have no trouble getting the money. The rate may be high, however, and the repayment terms stiff.

A cheap source of funds, if you haven't tapped it already, is the cash value in a life insurance policy. But if your only source of cash is a finance company at 20 percent interest or more, consider "borrowing" from the Internal Revenue Service instead.

The IRS says that if you can't pay your taxes you should, in any event, file your tax return on time, along with whatever money you can afford to send. You'll be billed for the taxes owed, at an annual tax-deductible interest rate of 12 percent, plus a penalty of one-half of one percent a month, nondeductible.

Getting an automatic extension for filing your tax return won't help much. No matter when you finally file, your actual taxes are due April 15. If you get a filing extension, and owe more than 10 percent above what was paid on April 15 you may have to pay a penalty.

A common reaction, when someone can't pay his taxes, is to put off filing the return entirely, without even asking for a formal extension. This, says the IRS, is the worst you could do. Late tax returns are penalized at a rate of 5 percent a month (nondeductible), up to a maximum penalty of 25 percent.

Some other tips for last-minute filers, from Sydney Kess of the accounting firm, Main, Hurdman and Cranstoun:

When time is short, and your income is eligible, it's tempting to file the easiest form, 1040A. But the short form won't let you take such lucrative tax-savers as the childcare credit, the credit for the elderly, the energy credit, the alimony deduction or deductions for contributions to Keogh Plans and Individual Retirement Accounts. All these items can be claimed in addition to the standard deduction -- but only if you file the long form, 1040.

Hasty filers may lose quite a bit of money by choosing the wrong filing status (married, filing jointly or separately; single; or head of household).

If your spouse died last year, you can still file a joint tax return for 1979. You can also use the joint-return tables if your spouse died in 1977 or 1978, and you have a dependent child in your care. If you're separated, but not officially, you can file jointly if your spouse agrees to sign the return.

You're single if you were unmarried or legally separated on the last day of 1979. But even the unofficially separated may file as singles if they lived apart from their spouses for all of 1979, paid more than half the cost of maintaining the household, and had a child in their care more than six months, whom they claim as a tax deduction. (If they had the child for the full 12 months, they qualify as head of household. Otherwise they're stuck with "married, filing separately," which is the costliest category.

Unmarried or officially separated people get the tax saving head-of-household status under the following circumstances: you keep a home for a child and pay more than half the cost; you keep a home for certain other relatives whom you claim as tax dependents, and pay more than half the cost; you pay more than half the cost of maintaining a dependent parent in his or her home or a nursing home.

Upper income people sometimes make the surprising, and expensive, mistake of overlooking the 50 percent maximum tax on earned income (generally defined as wages, bonuses, fees, commissions and pensions). This item applies to anyone reporting a taxable income below $60,000 on a joint return, $44,700 if you're head of household, or $41,500 if you're single. The 1040 form does not give you this tax break automatically. You also have to file form 4726.

You have until April 15 to establish an Individual Retirement Account and make a tax-deductible contribution. The quickest way to do this is at a bank or savings and loan (be sure to ask for their highest-paying certificates). You're eligible for an IRA if your employer doesn't cover you with a pension plan. The maximum tax-deductible contribution is $1,500 a year, or $1,750 in a joint IRA with a non-working spouse.

Keogh contributions also may be make up to April 15, but you have to have started the plan by last Dec. 31.