When in flight, smoke and sparks come out of the rocket end of the ships ... and the smoke goes out and up, while the sparks go out and down.
I still have fond memories of these buck-rogerian ships with their sparklers for atomic motors. Another convention, observed to this very day in the movies, is to have all space ships oriented in space so that up is in a single direction. In fact, there must be a Galactic Standard Office whose business it is to chose one of the infinite points and vectors in space and say "this is up, dammit!"
