IT'S the same every August. Before some of us are quite ready to let go, summer starts to slip away. In some ways, it's subtle, like having to throw a blanket on the bed at night or grab a sweater before heading out for an after-dinner walk. Outside offices, shadows from buildings and trees are growing longer, now that the sun is no longer directly overhead. In other ways, the change is anything but subtle, like the back-to-school advertisements that blare from TV sets and jump off the pages of newspapers, urging parents to hurry up and buy. Summer's end is easiest to see in store windows, where mannequins dressed in linen and cotton only yesterday are now wearing wool, tweed, and corduroy. Just as it's startling to see Christmas lights and artificial trees gracing store entryways in early November, it's jarring to realize that sandals, sundresses, and seersucker suits will soon be, ''like sand between the toes,'' only a memory. Actually, we shouldn't be so quick to dismiss summer. The days may be getting shorter and the nights a little cooler, but there is still time to sit on a porch and listen to the crickets. There is still time to catch an outside concert. There may even be time to put off, for one more day at least, that back-to-school shopping.