Arty's, a new restaurant and carryout, opened recently at 13476 New Hampshire Ave., in Silver Spring's Colesville Shopping Center. Mr. and Mrs. Robert L. Kieckhefer decided to check it out.

Mrs. Hieckhefer gave Arty's high marks. The food was good, prices were low, and employee attitudes were a pleasant surprise.

Mrs. K. remarked to her husband, "What a refreshing experience this is. Everybody who works here is so pleasant and polite and cooperative."

"Give them time," responded her cynical husband. "They don't understand the business yet."

I hate to admit it, but there is good reason for Bob's cynicism. I can think of a dozen local establishments that offered excellent values and extraordinarily good service when they first opened up shop -- and then went steadily down bill thereafter. I can't understand why owners and managers permit this to happen.

The low prices offered by new stores and restaurants can be charged off to advertising. After the new venture is well established, it must raise its prices to prevailing levels in order to make a profit.

But there is no good reason for abandoning the courtesy and friendliness that characterize a new venture.Attentive clerks and good service will win as many steady customers as good prices will. If I were Arty, or any other owner or manager, I would give top priority to reminding my staff about that. Frequently.

Incidentally, Bob Kieckhefer has reminded me that the District Line column will observe another birthday tomorrow. It first appeared on Jan. 13, 1947, so today closes out its 34th year. If I live long enough to write a column for tomorrow, it will be the first of the District Line's 35th year. I am not making any bets on that proposition, either way.

Shortly after the first of the year, I said to a colleague, "I've been feeling kind of dopey lately and I may take a few days off next week."

"Since when," he asked archly, "has feeling dopey been a handicap in the column-writing business?"

When I returned to the office after a week, I encountered the standard questions about where I had been and why I had been there.

This creates a problem. These days, it is assumed that anybody who does not show up for work is ill or on a trip. Inasmuch as people are seldom interested in hearing about the routine aches and pains of their acquaintances, I have learned that it is better to fib about minor indispositions. But this also causes trouble. The response, "Oh, I just took a week off," inevitably leads to "Where'dja go?"

Ordinarily, I answer honestly and say I just stayed at home, which astounds most people. When the question was asked once too often on Saturday, I startled myself by blurting out, "I went to Tibet."

My friend nodded. "That's a nice trip," he said, "but I always come back very tired."

I have now resumed my practice of answering honestly. I'm just no good at gamesmanship. SWIFT COURIERS

Richard Hughes of Lewisdale, Md., was in the Adelphi post office at 9:30 a.m. on Dec. 26 when a Christmas card was delivered to him. The card had been postmarked in Fairbanks, Alaska, at 7:30 on the morning before.

"That's remarkable service," Richard says. "I wish that same dog team delivered all my mail."

Richard, it may interest you to know that your report to me was postmarked "Silver Spring, Dec. 31, 1980," and was delivered to The Washington Post on Jan. 7, 1981. I don't know about your incoming mail, but I think your outgoing letters are already being handled by dog team. WALK, DO NOT RUN

Comedian Danny Klayman is in Costa Rica working in a film. His letter took one day to get from Costa Rica to Washington.

Danny reports, "I just got a call from Dean Martin. He said, 'Hey, if you don't hurry up and get over here to my New Year's Eve party, it's going to be over before you get here."

Don't panic, Danny. You can hitchhike back to the U.S. and still make Dean's party in plenty of time. ECONOMIC NEWS

Don Epperson of the Texas Tourist Council grouches, "The trouble with the business cycle is that there just aren't enough people pedaling."