My answer, and this pertains to skirts too, is to keep them for a bit longer and accessorize with fall tweeds, a moto jacket, combat boots, and a scarf. A cardi in a textured stitch is also great looking over a white tee. Of course this assumes that the shorts or skirts are of sufficient weight and do not scream summer, like seersucker or lightweight plaids. Rest easy, your favorite cut off shorts can stay a while!
My whole outfit, except the boots and bag, is from American Eagle and I took advantage of a storewide sale of 40% off. I love the needlework on my cut off shorts, and I like the way the combat boots contrasts with this feminine detail. My boots are from Payless and are really comfortable. The bag is Cole Haan.
Another option is to add a cap or a hat, barrowed from the boys. Mine is from Macy's and has a glen plaid trim . A cute hat helps us stay warm as the temperatures dip, especially at nights, when that nip in the air says September is near. A leather vest is also a good way to layer up your shorts for fall. The perennially perfect white shirt is another easy option. Add a plaid, thermal tee, and stay on trend while summer draws to a close.