Jot down the names that come to mind of people who can support either your
version of liability or your assessment of damages. Then, once a day for
three days, quiet your mind in meditation or relaxation and see if a few
more names don't surface. As soon as they do, write down the name and what
you think they can testify to.
Liability witnesses do not have to be witnesses who were at the scene. They
can be the neighbor who saw your brake lights working when you backed out
of the driveway (if the driver in the car that hit you from behind alleges
the lights were not working). They can be the friend who saw someone slip
on the same defect or wet floor or worn carpet step six months ago. They
can be the friend of your sister who commented to the apartment manager
that the steps (or deck) felt kind of spongy. They can be the neighbor who
has witnessed previous episodes of a dog viciously barking at the end of
its lease at passing children, etc.
Of course, if there were witnesses at the scene, you will want to get in
touch with them as soon as practicable.