Use battery negative as ground
I'd wire the set-up like this: -



Ground (pin 8 typically for an L298 but will be different on other devices) on the H bridge becomes the star point for the connections to the rest of the "system". This means that motor load currents are kept away from the arduino - these can cause problems. Also keep the wires to the motor battery and to the motor as short as you can to avoid induction into other circuits.