Get Grounded
Probably the most important conductor in a control system is "ground" or circuit common. This is the conductor to which all other circuits are referenced and in an ideal world it would have no electrical resistance or inductance. But alas, in the real world it does, which means there are a few things we have to do to make sure that our control systems work as expected.The best approach is to wire all of the common or ground wires separately back to the power supply terminal. This is called a star point grounding scheme as all of the circuit ground wires "star" out from the power supply ground or negative terminal. With this scheme, if you are measuring an analog signal and in the same control system, switching large currents, the voltage drop in the ground wires for the large current loads does not flow in the ground wire for the analog measurement, thereby avoiding changing the analog reading.
Using a star point ground system can also stop the "it does something strange every now and again" type problem, by routing noisy currents away from more sensitive circuits.
In general always keep your ground wires as short as possible and use the thickest conductor that is both practical and of course matches the circuit current.