Never put the keys on the car seat while you gather your things.
Invariably you will forget them as you close and lock the door.
Done that.

Though my car, like (most?) (all?) other cars, can be locked by
using a button or lever when exiting, I "never" (knock wood) use
that means of locking; I use the remote lock function of the
electronic fob that came with the car, and thus "almost always"
have both of those locking means in my hand before I close the
door. I would never, these days, buy a car without the remote
access function built in.

When I have, in the past, locked myself out of the car, the keys were
to be found not on the car seat but in the ignition slot.
That's how I learned always to use the remote access/exit fob.

Dave Oesterreich