A denim jacket is a perfect transitional piece. Denim is primarily made from cotton, which makes it much more breathable, thus making it a great accoutrement to a sun dress. As the seasons change, the weather may stay warmer during the day, but drop quickly at night. Carrying a light denim jacket with you will prepare you for finicky weather patterns.
Tights are thee go-to transitional accessory! They're thin, yet add extra warmth, so you can layer them with just about anything! My personal favorite way to wear tights is with shorts! I am not a big shorts fan by themselves, but I love wearing them in the fall with tights! You can virtually take any summer look and add tights to make it fall friendly!
Scarves are another accessory to help ease you into autumn. They go well with everything, but add an element of coziness to any ensemble! I like to pattern mix with scarves, printed dresses and tops!
Sweaters are kind of an obvious answer to this problem. Shoot I wear cardigans year round in LA! Instead of worrying about checking the weather everyday (unless you're anticipating snow), just make sure to have a sweater handy in your car or purse. I leave a sweater at work, because I am always freezing! And the rule of thumb for open toed shoes is that they're a go as long as it's over 65 degrees outside!
The last key transitional piece, is the boot. Damn, the boot is such a fabulous shoe. I love wearing boots with just about anything! Whether it's a frilly dress you wore to your cousins summer wedding, or your favorite pair of shorts, or just a relaxed pair of boyfriend jeans... put a boot with it!