For cutting thin plywood with a circular saw lay a
sheet of the
cheapest foam insulation board on your work table (I
use my folding
cutting table), then lay the plywood on top of that.
Set the saw depth
just slightly deeper than the plywood is thick and
cut away! The
plywood will be evenly supported and the foam won't
slow down or dull
the saw blade. The foam is sacrificial, but will
last for many, many
cuts if you're careful storing it.

I thought up neither the folding cutting table nor
the foam trick, I
heard about them somewhere...

John Kohnen