And following on the bike flags, when in the Coast Guard I taught a lot of guys to back boat traiers in, and something i did with some when we were using box trucks with hazmat equipment was to have short (about 2 foot) sticks painted with a red stripe that could be attached to the trailer sticking straight out to the sides. By painting the red stripes where you could just see them in the mirrors when the boat was straight in line with the truck it helped ensure you knew when the boat was moving out of line, and we attached them to the fenders so you also knew how far your trailer was into the water.
