Slowing motors down with resistors can be problematic. Heat generation is one issue, both generated by the resistor and within the motor. Lowering voltage is going to have a profound effect on the torque output of the motor, with the motor's physical load remaining the same, the windings will run hotter. Also the motors stall speed will be much lower, depending on the load, the motor may not be capable of turning the load with reduced torque output, or may fail to spin up, from a stop, under load.
Usually you can get away with resistors for very small changes in speed, but trying to half the speed will likely give you the troubles mentioned above. So much depends on the motor specs and the load it's running.
Really the best way to control motor speed would be a Pulse Width Modulated scheme. Actually PWM motor controllers aren't very complicated and are pretty easy to build, there's schematics available online for a build-your-own, kits you assemble yourself or pre-assembled controllers. Here's one example:
http://www.electronickits.com/kit/complete/motor/ck1400.htm (note: I don't know this specific product or vendor, just an example plucked from a google search)
PWM controller is probably your best bet.
D