What you can't put down your disposer. Vegetables, vegetable peels, fruit, fruit peels, pastas, egg shells, rice, bones, anything that sticks to your hand when wet.
European shower valves
Every two years European valves should be cleaned and lubricated. Every 10 years replace cartridge and pistons/screens.
Fill sinks to top with hot water and pull stopper with faucet still running. once per month, for any sink. bathtubs and showers use a good hair catcher and call a plumber when it slows down.
NEVER USE LIQUID DRAIN CLEANING PRODUCTS!!! THEY CAN DAMAGE YOUR DRAIN DESPITE WHAT THE BOTTLE SAYS.