PWM is the only method.
Roughly.
Torque in a DC motor is propotional to current.
Speed is proportional to applied voltage.

Hence, if you load your motor and it draws from a supply with a high output impedance, the volts will droop and therefore so will speed as you increase the load torque, since the additional current to feed the torque is trying to pass through your series resistor.

PWM.

DON'T FORGET A FLYWHEEL DIODE !!

Steve