At the beginning of the year, it’s a good idea to wipe down your locker with cleaning wipes, as you never know what was there before you. In the middle of the year, clean out everything, scrub down your locker, and, if needed, get rid of the smell of your gym clothes. Hanging car air fresheners on the hooks help a lot with any such smell.
Locker decorations can usually be found at office supply stores like Office Depot or at department stores like Target.