Tip #1: Since fall means back-to-school for so many, adding a little collegiate detailing to your summery staples will make it look right at home. Try argyle, plaid, bows, masculine fabrics, ties, and blazers to get this look.
Tip #2: To bring the weightless of your summer clothes down to the earthy feel of autumn, think color and texture. Bring out the darker colors in your patterned florals with a matching solid sweater or jacket. Add rustic elements like suede, leather, and metal. But don't be afraid to keep a little femininity. You don't want your summer item to look out of place, so add one other weightless item to keep the look cohesive.
Tip #3: Big slouchy layers and sophisticated palette will always evoke the mystery of fall (Halloween is only a few months away, after all!). To balance out a mini-skirt (and stay warm), an oversized sweater does the trick. Tights keep legs from getting chilly and an animal print keeps things fun. Try a pop of jewel tone for an unexpected touch.