How do I convert a .dvi file to a .ps for saving or printing?

Use dvips.

dvips -ooutput.ps input.dvi

dvips -omathematics.ps mathematics.dvi

Make sure there is no space between the -o and the output filename

What if I only need a few pages converted?

dvips -pstartpage -lendpage -ooutput.ps input.dvi

dvips -p3 -l12 -omathemathics.ps mathematics.dvi