The worst part about business travel has to be filling out expense account forms. Around my office the accounting department hounds us with dire threats to force us to sit down and write them up.

Only slightly less burdensome is then trying to keep track of how much money in my checking account is mine and how much belongs to the company from reimbursements for expenses charged to my credit cards but not yet paid.

Expense It! from On the Go Software in Marina del Rey, Calif., provides some relief if you have an IBM or compatible computer. While it may not be worth it for an occasional traveler, it is useful for a frequent traveler, small-business person or large company.

The $130 expense accounting program lets you keep track of business expenses, assign individual expenses to individual clients and projects and bill expenses to clients. You can print expense reports using either the standard forms provided in the program or customize reports to print onto just about any form your company uses.

Expense It! also helps you keep company and personal funds separate. If you set up a separate checking account from which to pay company expenses, the program will keep track of the amount needed in that account as you do your expense reports. Even if you don't, it tells you how much your company owes you and, if you keep it updated, how much has already been reimbursed.

You can run Expense It! on a portable computer with a single 720-kilobyte floppy drive, the typical configuration of a low-end IBM-compatible laptop, or a desktop model with dual 360-kilobyte floppy drives, or any model with a hard drive. It needs 640 kilobytes of operating memory.

Before you can use Expense It!, you have to do some set-up work to make it match your needs. It comes with eight defined expense categories, satisfying Internal Revenue Service reporting requirements. They are hotel, meals, auto, travel, rental, phone, entertainment and miscellaneous. You can change any or all of those and add as many as 16 more expense category names. The program lets you control how it will summarize and subtotal expenses.

These days the IRS allows you to deduct only 80 percent of the full cost of meal and entertainment expenses. The program can change that setting and the expenses to which it applies if the IRS changes the rules.

Automobile expenses can be handled in several ways. First, there are three choices of auto expense categorization. One computes mileage at whatever rate your company uses for reimbursement (but it allows only a single rate). Another method provides a two-tier rate, such as 24 cents a mile for the first 15,000 miles and 11 cents thereafter. Until this year, the IRS had such a two-tier system for self-employed persons. The third method is for people who get a flat rate per month for personal car use. It doesn't compute a mileage rate, but does keep track of mileage to compute the percent of business use at year-end.

Any of these three calculation methods can be applied to any of 23 separate vehicles that the program can track.

People usually pay business expenses in a variety of ways, and Expense It! allows you to characterize each of those with a descriptive payment code to distinguish between cash, traveler's checks, the company credit card and your personal credit card. As many as 15 payment codes can be established.

Entering expense data is very easy. A form is displayed on the screen and you simply fill in the blanks. When the cursor arrives at a blank field for which you have established codes -- expense type or payment type, for instance -- a small window pops onto the screen displaying the codes. Either type the first letter of the appropriate code to select it or move the cursor to it and it is automatically entered into the blank field. You can scroll through the full list if there are too many codes to be seen at one time. Note, however, that codes are not listed alphabetically unless you entered them into the list that way when you created them.

Creating a custom report to print expense data onto a company form is the most tedious task in the program. But you only have to do it once.

If a company were to adopt Expense It!, however, it would be a simple matter to create a preprinted form and the program template for it together to get a perfect match.

Finally, all of the data stored by Expense It! can be transferred into other programs, such as spreadsheets or databases, for further use.

Richard O'Reilly designs microcomputer applications for the Los Angeles Times. Readers' comments are welcomed, but the author cannot respond individually to letters. Write to Richard O'Reilly, Los Angeles Times, Times Mirror Square, Los Angeles, Calif. 90053.