ANYONE who thinks baseball still ranks as America's national pastime hasn't visited suburbia on a summer Saturday lately. Here, during mornings at least, increasing numbers of adults have been turning their attention to another peculiarly American institution, the yard sale. Preliminaries for this unofficial sport begin on Wednesday or Thursday, when suburban weeklies run columns of ads: ''Saturday, 9-2. Moving sale! Toys, furniture, housewares, and much more! No early birds.'' On Friday, telephone poles on busy streets fill with colorful hand-lettered fliers: ''Saturday - Multi-family sale! Good stuff! Turn right on Greendale.'' Well before 9 on Saturday, the first customers, ignoring pleas against early birds, pull up to a promising yard. Feigning boredom, they cast a practiced eye over card tables on the driveway and battered furniture on the lawn, hoping for a ''find.'' Every sale, however small, leaves both parties satisfied: One less item for the seller, one more bargain for the buyer. If our middle-class town is at all typical, the summer of '95 could go down as a banner year for yard sales. It is as if the urgent message floating through the suburban air has been: Clear out. Pare down. Simplify, simplify. That message is made even more insistent by the sudden crispness in the late-summer air - the hint of fall that commands: Get organized - or else. For some homeowners, this new simplicity comes as a necessary result of corporate downsizing. A job lost, a move to a smaller home or another city requires domestic downsizing as well. For others, the divestiture is voluntary. Those who spend their days with overloaded schedules at the office - the result of fewer employees left to do the work - long to come home to a place of order and calm, free from overload. One friend, a Sri Lankan who has spent nearly 25 years in the United States, sums up his current attitude toward possessions in three words: ''Unload, unload, unload.'' For those of us who have never quite caught the yard-sale spirit, there is nothing like looking at someone else's castoffs - rusty golf clubs, dusty Christmas decorations, mismatched drinking glasses, faded paperbacks, old clothes, outgrown toys - to give us new determination to put our own house in order. All that domestic flotsam and jetsam also serves as a reminder of the anonymous advice: ''Have nothing in your house that you do not know to be useful or believe to be beautiful.'' Yet putting a house, fat with possessions, on a diet is no simple task. In addition to the time required, the exercise involves letting go - breaking the bonds of sentiment or practicality that hold certain belongings with the tenacity of superglue. The advantage of a consumer culture is obvious: a plenitude of choices. Yet that plenitude can quickly turn into its own disadvantage: a surfeit of stuff. Beyond its initial monetary cost, every possession exacts another price by making silent demands on its possessor's time: Dust me, clean me, insure me, store me, move me. Or: read me, listen to me, play with me, admire me. Minimalism will never be the fashion for most people. We like our gadgets, our appliances, our electronic toys, our trinkets, our souvenirs. Our choices define our taste. They say: This is who I am. This is what I like. Even for those determined to find new freedom in a less-is-more approach, temptations arise at every turn. Soon a new season of catalogs will fill the mailbox, enticing readers to buy. Soon another holiday season will arrive, urging shoppers to give. It is the American way - the revolving door of consumerism. Where would our garages - not to mention our crowded attics and basements - be without it? By making a virtue out of downsizing, the garage sale will be around for a long time as the suburbs' original form of recycling, offering at least the hope of domestic order and simplicity.