Cheapest and easiest way to slow down a motor

[KenWittlief]

another easy way to drop voltage: silicon diodes have a fixed 0.7V drop. If you have diodes that can handle the current, you can put them in series till you get down to the voltage you want.

BTW, the easiest and cheapest way to slow down a motor is to wack it with a sledge hammer,

and the 'right' way to slow down a DC motor is to increase the strength of the magnets. The stronger the field, the more torque it will have and the slower it will run (due to the EMF voltage).