Does anyone have any that they want to share? (I’m willing to jeopardise my secrets in hope of the fact that no one here knows where I live - not that I actually have much compared to a lot of people!)
Liquid 2-in-1 concentrated fabric wash and conditioner is a great space saver: I can fit 5 bottles in the same space I can fit a box of powder and a bottle of fabric conditioner. That means I can do 112 washes compared to the 65 I could get out of the powder (or double that if I had to rationed the dose).
Some of the kick boards of my kitchen units click on and off meaning that I can store bottles/cans there. The ones that don’t come off below the units of draws can be accessed by pulling the drawer out so again, another handy space 
I used to have an Aunt who hid her money in the pages of the 1000s of books on her shelves throughout her flat: she knew where it was (something contrived to do with the names of the author and school friends) but I have a terrible memory so will stick to the knicker draw for now for my money!