You can't go wrong with an eighteen inch reflective farm triangle:

Actually, there are 12 volt DOT lights at the bottom corners of this reflective triangle, but the reflection almost totally drowns them out.
I believe my bike is a "Slow Moving Vehicle". I hope I don't stir up a debate with the hard-line VC'ers.

Also in this photo, you can see the amber marker lights on the front basket, and the sealed beam headlight is on.