For those with homes built to keep heat in, not let it out: aluminum foil your windows, shiny side out. Cover the window, tape up the sides, maybe tape cardboard behind it to help insulate from the heat outside and absorb heat that gets through the foil.
Love, a former Californian, whose had to keep houses cool without any electricity.
P.S. this is a method endorsed by FEMA and the Department of Energy, so if you need more than anecdotal evidence, there ya go.