For hand signals, I use the standards Left turn, Right turn (just like my mom did in a car when I was little) and one non-standard one--point my left index finger down and over to the left to show I (or "we" on the tandem) am headed right there. Like for a merge to get out of a right turn only lane or something.

I figure if car drivers don't know what I mean to do then they'll at least give me a little space to do it in.