DC motor as a generator: characteristics
A typical DC motor may be reasonably accurately modeled as an ideal motor in series with a resistor and inductor.  At any moment in time, the voltage on an ideal motor will be proportional to the rotational speed, and the current proportional to the magnetic torque.  The proportionality constants for speed and torque will be such that one watt of electrical power will yield one watt of mechanical energy.

When trying to use a motor as a generator, if shorting its leads doesn't remove enough mechanical energy from the source, you'll have to add a gearbox or use a different motor.  Trying to extract maximum energy from a given source of mechanical energy may be tricky, but if even a dead short doesn't result in enough mechanical loading there's no need to work things further.