Susan Johnson has her weekly routine down pat. Every Wednesday, she said, "I go through the newspaper's food section, clip out coupons and tear out ads and plan my menus from that."

The next morning, "I go to the cash machine and pull out $80. That's our allowance for the week. I buy groceries right away and whatever is left over about $20 to $30 is our fun money."

As the budgeter of her Fairfax family, which includes 1-year-old Olivia and 3-year-old Michael, Susan spends carefully, making sure she carries as little cash as possible. "I'm one of those people who, if I have cash in my hand, I'll spend it."

Unlike her grandparents who pay cash for everything, Susan relies on credit cards for the bulk of her buying.

She and Tom, who are both 28, would rather buy gasoline with a credit card -- and give up any discount they would receive if they paid cash -- than carry gas money around in their pockets that they might spend on something else.

"If I have cash in my pocket, it goes for soda or donuts" or for drinks with the boys after work, Tom said.

Tom works rotating shifts in the operations center of the international satellite consortium, Intelsat, testing different earth stations to make sure the satellites are being used properly.

Despite their beliefin spending prudently, the Johnsons frequently find it difficult to live without credit. "It always seems like a constant battle to pay off" the monthly bank card bills, Tom said.

Sometimes "we eat noodles for two weeks. We've had steak maybe twice this year, I don't think we've had roast beef."

Making ends meet every month will become even more difficult next week when the Johnsons expect to sign an $82,600 note for their three-bedroom townhouse in Fairfax. They will put up about $2,000 cash as a downpayment, the same amount Susan's father put down years ago when he bought an Arlington home for $26,500.

Susan and Tom's monthly housing payments will rise from the $625 they have paid as renters to about $1,000. "It's a little scary," Tom said.

To meet the higher payments, the two will have to drastically pare back on the $420 they have been setting aside monthly in Tom's credit union for savings.

"When we first started out, we were paying $275 a month for a one-bedroom apartment," Tom said. "That was five years ago. Rent's gone up from $275 to $625 -- it's doubled, and of course our income hasn't. So this is a hedge against inflation -- and a hedge against future increases."

Until a few months ago, the Johnsons didn't have enough money to afford a downpayment on the house. But thanks to the restructuring of Tom's pension plan, they were able to come up with the minimum downpayment required for an FHA loan.

Even with the runup in credit card bills, the Johnsons said they had no problem obtaining a mortgage, partly because they had no other outstanding debts -- no student or car loans, and also because they have a good credit history.

"We had the money at the time" of every car purchase, added Tom who noted that the couple has bought only used cars.

"Besides, he added, "we never made that much to make the interest deductions from any loan that important in reducing taxes ."

Susan acknowledged that she could help ease the cash-flow crunch the family now faces by going back to the job teaching school that she held before she had Michael.

"I have friends who do work. They all have nice houses and nice clothes. I like nice things, but at this point, it's more important for me to be home."

Tom joined Intelsat four years ago after being laid off as a construction supervisor. "Back in 1981, when the housing industry went to pot, I lost the job with the construction company where I was a superintendent.

"I went to work for myself, doing a bunch of things that weren't very pleasant -- digging ditches, installing home installation. That lasted about 9 months, and it was just time to get out of that.

"I took a job as a clerk for Intelsat just to get into an organization where I could get some health insurance, and we could could start a family."

Since then, he's had several promotions. "At this point, I don't see any reason to leave," Tom said. He has applied for one of the three space-shuttle slots slated to go to Intelsat employes beginning June 1987.

The Johnsons live comfortably, with the townhouse full of many of the modern conveniences found in most homes, including a stereo, color televison and a video-cassette recorder. Vacations -- including one to England -- "have kept us poor for the last two years," Tom said.

Yet, Tom points out, they don't have the most up-to-date consumer goods. "There are some people I know who are like little kids; they have to buy the newest toys. If a VCR comes out in stereo, they've got to have it. If an IBM computer comes out, they've got to get it.

"In that respect, I'm not as materialistic as a lot of people might be our age. We don't drive flashy cars. Conspicuous consumption is not our strong suit."

While their grandparents have none, Susan and Tom have 14 credit cards. "We've got store accounts at Woodies, Hecht's, Bloomingdales, Lord & Taylor, J. C. Penney's and Sears. And we've got lots of gas cards -- Exxon, Mobil, Shell, Amoco, Texaco." They also have three bank cards.

"This last year has been the worst for us as far as credit because we charged too much for Christmas last year," Susan said. Expenses also mounted because their car needed several repairs.

"We kind of had to carry over our VISA forever. We finally paid it off at the end of October," she added.

Unlike her parents and grandparents, Susan often writes checks knowing that the money is not yet in the bank but anticipating its arrival by the time the check is cashed.

"I'm a big floater," she said, adding that only once has she had a problem being overdrawn, and that was due to a mathematical error.

Until now, Tom has been saving $210 out of every paycheck twice a month. Those savings will be diminished with the increased cost of housing.

But he plans to continue saving 6 percent of his salary -- half of which is matched by his employer -- in a tax-deferred employe savings plan.

"We've never really had enough extra to put aside" for other savings and investments, Susan said. They hope to be able to make other investments after digesting the added cost of home ownership.

In the meantime, she said, "we seem to be doing fine the way we're doing."