Easy. You put some shampoo on your head before setting off to the showers, and that way you never leave your valuable shampoo in the shower block ever again. Of course, you'll look a bit of a nit walking to the shower block with gloopy shampoo on your head. So, the other option is just to go to the shower block with no shampoo at all, and look into each cubicle until you find someone else's shampoo that they've left behind. This always puts me in an excellent mood for the rest of the day, or at least until i get back to the boat without the flippin towel.