I wouldn't say I decorate as soon as they go back, 100 degrees doesn't scream fall to me. That being said. I would hit Michael's and At Home for the decorations. I saw lots of fall stuff when we were at At Home getting stuff for my daughter's room.
It is awesome and HUGE!! There is a section for every room of your house, and tons of seasonal. They also have a large clearance area. If you sign up for the rewards program, you get coupons. If I remember correctly, if you sign up for the program before you go in, they will send a coupon via email.