Another thing people often do wrong is running the power and ground wires separately, rather than as a
closely spaced pair (twisted pair is ideal). The current from power supply to power and ground are
equal and opposite so running them together cancels out any tendancy to radiate noise/interference.
The absolute worst case was as above with one wire formed into a coil. Coils radiate, they are efficient antennas. You can get away with coiling up a twisted pair though as the currents all cancel.