Using a pulse-width modulated signal to possibly reverse polarity
If you have an H-bridge controller that can switch cleanly at a PWM frequency which is sufficiently fast relative to the motor's inductance (the lower the inductance, the faster the PWM must be), driving it with a waveform that's 60% forward and 40% reverse will be a good way to drive it forward at 20% speed; 40% forward 60% reverse will be a good way to drive it backward at 20% speed.  If both conditions above are met, driving a motor in this fashion will give a speed response which is much more linear than PWM'ing between driven and "open-circuit", and will also be more energy-efficient.  Additionally, trying to drive the motor at a speed which is somewhat slower than it's presently turning will provide regenerative braking [i.e. allow motor energy to be fed reasonably nicely into the supply].

The important thing to note is that running the PWM too fast for the H-bridge controller may waste energy in the H-bridge controller; running it too slow for the motor inductance will increase the amount of energy wasted in the motor.  If the PWM is much too slow, driving the motor at half speed may use many times more energy than trying to run it at full speed.  If, however, the motor is driven with a fast PWM and the H-bridge can handle it, efficiency may be very good; a stalled motor driven at 75% forward 25% reverse will have about half the torque as would one driven at 100% forward, but will only take about a quarter of the power [about 75% of the time, it will draw about half as much current from the supply as it would if on 100%, and the other 25% of the time it return that same amount of current].