Here are some phase portrait generators for a few Hamiltonian systems, and the programs are written in . . . Postscript! The postscript files contain the numerical computations along with formatting routines, so all you need to do is send the file to the printer and let it do all the work.
Each file includes documentation in the comments, and there are a number of parameters that can be set at the top of the file. The parameters should be self-explanatory, but one parameter of note is the color vs. black/white option, which should be set correctly, since the color setting does not look as nice on a greyscale printer. Additionally, for the continuous-time systems, the integrator method and stepsize must be set to ensure an accurate phase space.
Note that these files can take a very long time to print, especially on older printers. For example, the standard map file with the settings here takes 6 minutes on a Hewlett Packard 4500N color laser printer, 17 minutes on an Apple Laserwriter 12/640 PS, and 45 minutes on a Hewlett Packard Laserjet 4MP. You can reduce the time to print by selecting fewer points for output or rasterizing the postscript on a fast computer (using Ghostview or Acrobat Distiller) before sending the job to the printer.