We have covered this before - dropping volts at low power is probably best done with diodes in series.
But if you want a motor to move very slowly you probably don't want to drop the volts - to get a very slow speed you may need such a low voltage that the motor can't overcome bearing friction.
In this case you use pulse width modulation. You send a signal which has full volts for, say, 50% of the time and 0 volts for the rest. That gives you 'half power' - but the motor is given enough volts to be sure of getting it moving.
This technique is used by the model railway boys for slow speed when shunting. So... something like this?