The resulting file should begin with %!PS. If it doesn't it's not postscript. Remove any characters that are before %!PS, and (to be on the safe side) remove anything after the final %%EOF line.
If you have problems on a Macintosh, check that you're using the LaserWriter
8 driver. Platform-specific considerations do crop up. The EPS generated on the
Macintosh will use ASCII 13
line terminators, while Unix will use
ASCII 10
(and DOS will use both). If this causes trouble, use
emacs to convert, or try (in Unix)
tr "\015" "\012" <original.ps >new.ps