Getting back into the habit of doing schoolwork, studying, and preparing for classes is difficult to do after a long summer of fun. It's easy to put off preparing for school when school isn't in session.
A good practice is to set aside at least 30 minutes a day to read. Try not to just read for pleasure. Find some topics that interest you and look up scholarly articles on the subject. Doing this will help you get back in the habit of thinking critically and engaging with higher-level subject matter.
Additionally, try and do a little bit of writing everyday. It doesn't have to be essay length (or quality, for that matter), but try and at least write down some notes from your readings, writing a summary of what you read that day, or even doing a little creative writing.
A good habit to begin during the summer is to make a calendar. If you know you have a future obligation, write it down. This will keep you organized and will ensure that you won't miss anything you might have planned earlier. This is a great habit to have during the school year when you have multiple assignments due in multiple classes.
Try doing one or two of these everyday in the weeks leading up to school, and you'll be prepared in no time at all.