I have found this to be a common problem with boys (and men).
One thing that helped was to write out little cards of instruction for them to follow, and also have a whiteboard by the door with reminders such as 'Swimming kit or death' graphically written on it.
I had a major problem with my bathroom looking like it had been hit by a natural disaster after my boys had used the shower, so I wrote out a card like this:
How to have a shower-
1) Get out pyjamas or clean clothes and bring them to bathroom
2) Turn on shower switch from outside the door
3)Take out towel and put it near shower.
4) Take off clothes and put in (note the preposition carefully here - not on, or beside) the laundry basket
5) Turn on the shower and clean yourself and hair
6) Turn off shower, take towel you left nearby and dry yourself
7) Put towel in laundry basket
Put clean clothes on
9) Turn switch outside the door, off
Now we all know that girls do not need instructions such as these
. But they work brilliantly with boys!
All of us make mistakes/forget things/lose things sometimes. But we slowly learn to be more careful. Once ds1 left his games kit on the bus but I made him go to the bus station to get it back. Not a pleasant experience. He has been more careful since.
Edit - whenever I write the number eight, this forum inserts a smiley instead?!