Buy a backpack which is light and waterproof.
Always use both straps over your shoulders. Choose padded straps so that they don't dig into you.
Alter the straps so that the backpack fits up against your shoulders and back when you are standing up straight.
If you are having to lean forward, the straps are not tight enough.
Don't have everything including the kitchen sink in there!
Clean it out regularly – half dead lunches, moldy apples, etc. are extra weight as well as germ factories!
Give your backpack a regular wash or a wipeout especially at the end of the term and before each term starts. It will look and smell better and it will last longer too.
Only carry what you actually need. Lockers, trays, and desks are for storing books you don't need all the time. Organise yourself so that you have only what you need in your backpack.
Check your lesson timetable and only take what you need that day. Leave everything else home.
Have a different bag of sport's gear.