For general side visibility I have been looking at a few options:

1. I have been eyeing the Light and Motion urban lights that have the side yellow lights on the headlight and tail light. I have seen quite a few commuters using them on their bikes and the yellow lights are quite noticeable from the side.
2. A helmet light seems effective to "signal" to a car or pedestrian you are there, but unless you turn and look at them its not much more effective then normal headlights.
3. reflective tape, which only works well if there is light to relect.

As for the trailer, I saw the best trailer light earlier this week - it looked like one of those orange flags and poles that some people put on their trailers, but was really like a red led string light. Lots of lights waving in the air above the trailer made it very visable from back and side! Not sure if it was a stock product or they guy made it.