Great techniques discussed here.
I use a Blue Lantern Modular Subharmonic Generator to achieve this and have had great results. This module outputs a copy of the original harmonic and 3 lower harmonics and can also act as a clock divider. I usually use the triangle or pulse wave out from the Rat King Modular Tone VCO and pass the subharmonic generator's output into it's out filter and LPG/VCA. I use Maths to supply the envelopes for both the master VCO and the SH voice, giving me the option to create alternative envelopes for the subharmonic's VCA and filter.