My y11 DD has poor memory and organisational skills. Here are the kind of things we have found have helped:
DD1's bag is right by the front door. Very hard to open the front door without first picking up the bag. As it is taken every day, unlikely to be forgotten anyway.
Extra stuff to be taken in is physically tied to her rucksack, so to pick up her rucksack she also needs to pick up PE kit, dance shoes or whatever. If there is stuff in fridge e.g. lunch or food tech then physical notes by front door and at breakfast place. If necessary a big red tag tied to school bag.
Rules for on the bus - instrument case either physically linked to school bag, or must be kept on lap at all times.
Always check where you have been sitting when you leave: good for classroom, canteen, bus, and generally when out shopping.
Make sure random things like music lessons are written in planner, preferably in a different colour.
All messages to always be written in planner. Planner to be checked after school, before school, at school - constantly.
2 sets of stationery - one in school bag for school, the other where homework is done so school set lives in the bag.
Generally make sure everything has a home and it always always goes back there.