My trick to this is to leave the bottle on the tub/shower floor when I'm truly done with it. It increases the chances that I'll get rid of the bottle once I'm out of the shower. It does not necessarily guarantee that I'll remember to grab a new bar of soap or shower gel, but it interrupts my usual post-showdr flow of things enough to up the odds that I will
I used to do that too! For some reason it didn't work as well for me, though. I'd remember to take the empty bottle with me, but I'd forget to replace it before leaving the washroom