"The income tax," said Will Rogers, "has made more liars out of the American people than golf has. Even when you make out a tax form on the level, you don't know when it's through if you are a crook or a martyr."

business owners inevitably flirt with both roles at tax time, simply because the line that separates their own finances from their companies' is usually blurred. Who really benefits from the car or the country-club membership? Should insurance policies or the annual medical checkup be paid for by you or by your corporation? Most of the year, the answers don't matter. Come April, they can matter a lot.

The reason, of course, is that anything bought and paid for by the company typically counts as a business expense, and it is therefore purchased with before-tax dollars. Buy the same item yourself and, if you are in the 50% bracket, it will cost you twice as much. Since the Internal Revenue Service has an interest in making sure that what it considers taxable income actually gets taxed, it tends to inspect this question of who is paying the bills pretty carefully.

That, however, should not stop you from arranging your affairs so that the company is picking up as many tabs as it can. Although it may be too late for any new arrangements to make a difference on your 1983 return, it is not too early to begin planning for 1984's. And since some of the rules about what the company can and cannot legitimately buy on your behalf have changed in recent years, you need to make sure that your divvying up of the expenditures is up to date.

In general, say accountants and financial planners, there are three broad areas to consider.

Insurance. The first $50,000 of group term life insurance, as most business owners know, can be provided by the company, written off as a business expense, and not counted as taxable income by the employee. To supplement this coverage, many executives utilize a so-called split-dollar policy, under which the company pays part of the premium on additional coverage (and is entitled, on the policyholder's death, to get back what ever it put in).

An alternative, says William F. McMurry, president of Financial Profiles Inc., in Los Angeles, is to have the company buy your life insurance policies outright and enter into a deferred-compensation arrangement with you. That way, the company will be paying all the premiums you used to pay with aftertax dollars, yet your beneficiary will get the proceeds through the deferred-compensation agreement. In this case, of course, the policies become a corporate asset, and thus are subject to creditors' claims in the event of financial difficulties, so you may want to retain some coverage on your own.

You can also set up a medical reimbursement plan, with the consultation of your attorney, to pay for any medical expenses not covered by your company's group health insurance policy. Such plans can include both you and your family, and any benefits paid out under them don't count as taxable income. However, McMurry cautions, the rules on these plans have been tightened up. Any plan not written by an insurance company must cover all your employees (with only a few exclusions, such as those under age 25). From a personal standpoint, therefore, they are most valuable to the owner of a company with a relatively small payroll.

Travel and entertainment. Into this category fall items like the company car, tickets for opening day, and that business trip to Texas that included a side excursion to Acapulco. Into it also fall most of the biggest bones of contention. "It's probably the area that's most abused," says Joseph Gevock, an accountant with McGladrey Hendrickson & Pullen's Peoria office. "It's the one where everybody is aware of deductions and where the IRS concentrates most heavily."

In most cases, a car that is used mainly for business purposes should be owned by the corporation. A company buying car can depreciate it in three years and is eligible for an investment tax credit in the first year. You can use it for personal travel, Gevock says, but "you should make some sort of reimbursement [to the company] to avoid IRS problems. If you don't pay any reimbursement at all, you're out on a limb a little bit."

Other travel for business purposes, of course, can be fully paid for by the company. That includes not only the cost of transportation, for example, but lodging and meal expenses if the trip lasts overnight (or if business clients are entertained at a meal). A business trip can be safely combined with a vacation so long as the IRS judges that the trip was made primarily for business purposes and so long as the expenses are broken down accordingly. In general, any cost that would have been incurred if the trip were made for business alone -- accommodations, for example, and airfare -- can be included in the business portion and thereby deducted. There are special rules, however, for foreign travel. You can deduct only the cost of the cheapest-available airfare, and you must devote more than half the total trip to business.

Country-club memberships; season tickets to sporting events, and the like can be bought by the company so long as they are used for such business purposes as entertaining clients. You can also be reimbursed by your company for the cost of entertaining business associates or employees at home -- although not, ordinarily, on your yacht. For the company to get the deduction for a cruise, write tax experts Barry R. Steiner and David W. Kennedy, "you must submit proof that you engaged in business and that the main purpose of the cruise was the transaction of that business." In most cases, they say, the IRS "presumes that cruises on a yacht are made not purely, or in most cases even partially, for business reasons but mostly for pleasure."

The key to all travel and entertainment deductions, the experts agree, is good record keeping, and the best way to do that is to maintain a diary that includes receipts and notations of who you entertained. "Where you run into problems is when you say you don't have any records," says Bob Glovsky of Baystate Financial Services in Boston. "Receipts seem to get you out of almost all problems." But with good records, Glovsky says, "I would advise every business owner to take the deductions. You may be being aggressive, but you're not doing anything fraudulent. Take the deductions and argue them."

Cash flow. Like many business owners, you may draw a relatively modest salary and pay yourself a good-size bonus at year-end if your business's profit situation warrants it. Depending on your company's fiscal year, you may be able to save some money by delaying the collection of the bonus. Say, for example, your company agrees to pay a bonus of $10,000 before December 31, when its fiscal year ends. If the company is on an accrual basis, that's a deductible expense right then -- but if you don't collect it until January or February, you won't be liable for personal income taxes on the money for another year. The only thing to watch for in this connection is the legal doctrine of "constructive receipt," under which the IRS might argue that you, as controlling owner, could pay yourself the bonus at any time. If you spell out the agreement with the company in writing, you should be safe on that score.

Finally, that annual medical checkup is deductible right off the top. Whether or not you have a medical reimbursement plan, a company can safely order physicals for its officers without fear of running afoul of the IRS.