Go through everything before you put it away for the summer. Throw away, give away, or recycle classroom decorations, posters, bulletin board paper, etc. that you no longer want to use. Keep only one paper copy of each assignment, handout, and test that you think you might use next year. Recycle the rest. Pester students until they've picked up old projects still hanging around your classroom. The more you deal with now, the less you'll have to go through in the fall when you are juggling meetings while trying to get everything ready for a new school year.
Buy a flash drive and download all of your files from your school computer and server. Servers are not infallible and there's nothing worse that arriving in the fall and finding out that everything you left on the server (and its back-up) has disappeared.