Design for the stop, not the desk
Most apps are designed on a big screen in a quiet room and used on a small screen somewhere loud. The gap between those two places explains a lot of bad software.
Go where it's used
Before I draw anything, I spend an afternoon where the product is used. For a tram app, that meant standing at stops at rush hour. You learn things no interview tells you, like how often people hold their phone in one hand and a coffee in the other.
Test with one hand
I test every key screen one-handed, with the phone held low. If the main action is out of reach of a thumb, it moves.
Assume a bad signal
People open apps in lifts, tunnels and basements. Anything they need in that moment, like a ticket, should work without a connection.