Surprise your school's janitor or custodian and clean your classroom before they arrive.
If you are a teacher, motivate your students to be aware of their messes throughout the day, and then for the last couple of minutes each day have them really clean: wipe down their desks, sweep the floors, clean the chalkboard, stack the chairs, etc. If you are a student, suggest to your teacher and classmates that one day this week you would like to clean the classroom before the janitor gets there in the afternoon or evening.
This is a great way to teach your students kindness in a "hands on" way. Your janitor and custodian will appreciate this generous gesture as well.
Leave a treat for the person who regularly cleans your room. Leave them a note letting them know that you've gone ahead and cleaned it for them, and encourage them to take a break and enjoy the snack you've left them. You could do this once a month and have different students volunteer to provide the snack each month.
Every little bit helps; if you don't have time to clean the whole classroom choose one thing you can do. Wipe down the desks, or do a quick sweep. Put books and art supplies away, or bring your trash to the dumpster instead of leaving it for the custodian to do.
Some tips for keeping the classroom clean.
Help your students keep organized desks.