CALL IT what you will -- garage sale, yard sale, basement sale -- it's a latter-day phenomenon and a sign that summer has finally come. For as sure as the air turns balmy, garage sales spring up as if properly mulched all winter.

This may well be the year for the yard sale you've been thinking about for so long. Treasure or trash, surely you have enough around the house that you're eager to dispose of, trash that somebody else may want.

Even if you think you don't, start hauling out the stuff you do have. Old bowls, bookends, and baskets you'd forgotten about will come to light. Picture frames, ash trays, and candle sticks -- games, suit cases, and books. All these go in your yard sale, and all are saleable.

The amount of money you clear will depend upon how much of value you have to sell. Yet even with only a few knickknacks, you can convert unwanted items that are using needed space into a fairly good financial return. Your large items will bring in the most money. Furniture, electric appliances, musical instruments, and antiques are the most desirable items. Shoes, used clothing, and curtains, the least.

The first thing to do is set a date. Give yourself at least three weeks to prepare -- maybe more if your time at home is limited. Avoid a holiday weekend, and don't worry about overlapping with neighbors. In fact, two or three sales on the same date in the same area are helpful, because potential customers are more likely to turn into a street that offers more than one opportunity to browse.

Garage sales will proceed well into the summer, so you don't have to be in any hurry, but once you've set your date, try not to change it. No one can guarantee a sunny day in June or July.Have a few large plastic covers available in case of sudden showers, and you're all set.

Plan to start your sale about 9 o'clock in the morning. From past experience, we learned that while a few people trickle in before 10, and after 3 in the afternoon, the greatest flurry of activity takes place during the middle of the day.

Don't attempt more than two days in a row. On both sales we held late last spring, we sold about four times as much on Saturday as we did on Sunday, on both occasions. Yet if I were doing it again, I'd still go for the second day. Once all your preparations are completed, it's just a matter of being available that extra day.

Pricing your articles for sale is an art in itself and should be an on-going project. The price must be just right for the article to move and still clear a little cash.

As you visit other yard sales, make notes about prices on items similar to the ones you will be collecting, as we tend either to down-value or exalt our own possessions. We're tired of them, so we can't believe anyone else would want them. Or we loved them so much when new, we can't believe they're close to worthless today.

Some of the most out-and-out junk in your home may be worth a fortune in some collection -- old, tobacco tins, the kids' discarded baseball cards, obsolete political buttons.

I'll never forget the large glass vase I priced at 50 cents. When my daughter-in-law looked over our display, she suggested that we raise the price.

"If anyone wants a vase," she said, "surely they'll pay at least $2 for it."

It seemed like an enormous markup, but on went the new price, and out went the vase the following morning to the very first person who came in.

"Say, Dorothy, look at this," the customer called to her friend, "this wonderful old depression glass vase for only $2."

Later I learned that I could have gotten $20 for it, or more.

Price your items as you unearth them, collecting them in one spot until the day of your sale. You can always change the price, or even remove it from the display, but getting things ready in advance will save a headache or two later.

In any variety store you can buy tags and stickers on which you can mark the price of the articles. And throw away nothing. That's the whole idea. It may be junk to you, but it could be a treasure to someone else.

After the sale, if something doesn't move (and a lot of stuff won't), then discard it. Or donate it to one of the charitable organizations that collect and resell all kinds of items; then deduct the value of it from your income tax return.

Tops on your agenda of essential things to do is getting your publicity ready, as it's probably the most important step you'll take toward a successful sale. You'll want signs, notices, classified ads, and handouts. A two or three-line ad in the newspaper is a must. Mention a few of your more impressive items, particularly with prices, if they are bargains.

If your community has a bulletin board or a newsletter, give yourself plenty of time to get your notice in, being sure to include dates, hours and full address. One of my neighbors put on a dandy sale one spring, had a whole bunch of handouts made, and forgot to put in the date.

If you have access to inexpensive printing, get a handout ready to distribute at the office, at your swimming pool if you belong to one, or at any other community club or meeting place.

Finally you'll want signs. Lavish your neighborhood with posters of all sizes and don't wait until the night before to start looking for materials. The bulk of traffic comes from signs, so plan them well in advance. The larger the sign, the more attention it will attract.

You can buy poster board, 22-x-28 inches, at any stationer's store for about 50 cents a sheet. Or, try to get huge corrugated boxes from the grocery store, the kind used for packing toilet tissue or other paper products. gThese make effective signs, as they are strong and sturdy tacked on lampposts and poles around your home.

Use large-tipped magic markers in vivid colors for your lettering, and make it bold enough to be read from a passing car. Unless you have experience in creating outdoor signs, you are likely to make them too small. What seems to be monstrous when viewed on your kitchen table is woefully inadequate when seen in the street from a moving vehicle.

Too many signs we see are illegible because the person making the signs didn't know how to "plan ahead" as the humorous little poster has warned. On a small piece of paper, scribble out, line after line, what you want your sign to say.

What you're doing is making a dummy or pattern for your real sign. Get it all down, remembering all the essential facts, and then attach your big pieces of board.

If your first line is to say GARAGE SALE, count the number of characters. The one in the middle is the 6th letter, or the letter "E" in garage. First draw the lines you plan to do your printing on. Now measure your board, and draw a line down the middle.

Starting with the middle "E" in the center, begin to print backwards. If you follow the pattern, you won't go wrong. You won't emerge an instant artist, but at least you won't end up squinching in the last few letters on the line.

If you place a sign at every artery that flows into your street, you will use anywhere from 10 to 15 signs, so start your sign-making early. Each one could take up to a half-hour to make, if you do them right. Get them up the night before the sale or early the following morning. Tack them at both top and bottom, high enough for passing motorists to see.

The day before the sale, get some change from the bank. You'll want about twenty $1 bills, a couple of $5's, and perhaps one or two $10's, depending upon the prices of your items. You'll also want about $5 or $10 dollars in change.

Finally, on the day of the big event, spread your sale area with tables on which you'll arrange your merchandise. Display like things together, and place small signs where necessary, on key objects. Try to have an outlet available for customers to test electrical appliances. We had two stereos in our collection, and we kept records playing until they sold. Not only did it prove the equipment was in good condition, but the music added to the general carnival atmosphere.

When it's all over, you'll have learned a great deal, had a rewarding experience, cleaned out your burgeoning closets, and added a few extra dollars to the family coffers.

Now go out and remove all those signs. A new sale is coming next week, and your neighbors will need the space.