Be practical. Get a little magnetic mirror and a magnetic caddy. A few sticky notes and stickers would look cute. You can get this from Walmart, Staples, Target, etc. Most major school supply stores.(:
If your lockers are metal, get magnets, perhaps pictures of your friends your going to school with, and put them on the inside of your door or the back of the locker. Basically anything will work ;)
during the back to school sale season target is where i would get a lot of nice stuff to decorate my locker. i am not sure if they have it year round though, you can check
Stickers, posters of your fave bands, lists of things you want, pics from magazines of celebs/cool clothes you like. A mini whiteboard, a mirror on the inside!
there is a store called 5 below old students would go there and pick up a few cool things...if u have one in your area if not then target
I got a lot of my back to school things at Staples on sale :)
stickers :) almost everywhere u find stickers.
Stickers, posters, duck tape, wall paper.