Apart from riding an orange bike and wearing an orange helmet, I have added a few gewgaws to make the bike more visible to distracted drivers, and to make them more visible to me.
First, I added running lights fore and aft. A white flashing light for the handlebars and a red one for the seatpost. They are not bright enough to ride by at night, but give the bike more visibility during the day, especially if I’m in shade.
In addition to the taillight, I mount a removable red light to the rear of the trailer. It just makes sense to me to have a light at the farthest point back my rig extends.
To make cars more visible to me, I added a Garmin Varia, which is a radar that warns of cars approaching from the rear. One nice thing about this unit is that it interfaces with the Ride With GPS app, which I use for navigating. It has both an audio warning (beeps) and a visual depiction of how far back cars are behind you. I used to ride with a mirror attached to my glasses, but found I spent too much time looking in the mirror. This way, I stay focused forward, and am not surprised when cars approach.
I tend not to like Garmin products; I find their interfaces range from clunky to non-functional. Happily, there are no settings. You mount this thing on your saddle and let it do its thing. True to form, the mount Garmin provided was useless. Apart from being cheap plastic, it mounts on the seatpost; having a dropper post, that wasn’t an option for me. Plus, running 29ers, I need it to sit much higher. After a lot of searching, I found a really solid, intelligently-designed mount from BUP Labs. These guys are hard to find–they don’t turn up when I search the web–but the mount secures to the saddle rails, has an O-ring in addition to the mount for extra protection; and just to be sure, includes a safety tether in case the quarter-twist mount fails. No way you’re going to lose your $200 radar.
Finally, I made a custom Pony Express flag to fly from the trailer. This probably isn’t as visible as the orange Burley flag that came with the trailer, but it’s much cooler. What it lacks in visibility, it should gain in trail cred.