"Is there a standard technique for halfing this voltage?"

Pulse Width Modulation is THE way.

A resistor to drop voltage is rather crude and wastes so much power.

Just mail me if you need the actual circuit using the latter. Have been using one such for DC motors for a long time. It just uses tow 555s and a handful of other passive components. The final driver is a MOSFET.

The other big advantage of using a PWM is the excellent low speed torque characteristics.

Raghu