A quick Google search should reveal the typical safety flag mounting bracket, which you can also see pictured here on the Bikesatwork site: Bicycle Trailer Accessories | Bikes At Work, but there are also clamp types.

I think the flags that have reflective material are the best type, and are, for some reason, most commonly found from recumbent specialists, e.g. Hostelshoppe, Bentrideronline, and Recumbentbicycles.

I think the reflective triangle, and really, reflective stuff in general, communicates 'slow moving vehicle' really well, but active lighting is great, too. I guess it's hard to really go wrong with visibility stuff, though obviously at some point enough is enough.

I have outfitted a couple of my trailers with active LED light bars which I 'repurposed' from an automotive wheel well lighting kit. They're weather proof, multi-color, and offer tons of illumination options, and they're even remote controlled! I do have to move the battery pack (a RadioShack 8xAA holder) and the control unit, both of which I keep in a single, tidy project box, between the trailers, each of which are equipped with two 3' (iirc) light bars, depending on which one I use since the kit only had one CPU, but for single trailer use, that wouldn't be an issue.