Hi-viz reflective trim the length of the trailer. Fluorescent yellow-green tape, e.g. Reflexite Daybright lime yellow, stands out better than plain silver or yellow, especially at dusk. That's why the color is now standard for pedestrian signage like crosswalk signs.

We also have two red taillights on our trailer, one on each side, to make it more visible that the trailer is wider than a single bike. The tail light on my rear rack reflects off the front and sides of the trailer, which also increases conspicuity.