I know many programmers tend to think of UML as "that stupid crap they want me to put in the document that is never going to be looked at", but it was actually designed to solve the problem of programmers communicating.
Know UML, even if though it rarely matters if you use an open arrow or a closed arrow because the fact is that it will confuse some people if you use the wrong one. Programmers are very single-minded critters and that's one of the things they often enjoy getting "Stuck" on.
Know a few basic UML diagram types. Everyone knows some level of object diagram, I often combine inheritance and containment diagrams in the same picture--don't be too strict.
Read a few flow diagrams and actually create one for a complex flow you're working on. They are so damn cool at analyzing what's going on and at communicating anything beyond a trivial single method call/return. I didn't know about these for about 1/3 of my career and was just dumbfounded the first time someone threw one out on the whiteboard (this was after I knew everything--but of course, every year I learn more and then decide I FINALLY know everything).
Finally--you're standing there talking with the person. Really a box on the white board is just something you can point at so that the next time you point at it the person knows that you mean the same thing... it'a a visual assist to enhance your verbal communication, that's all.
Edit:
This page is a good intro to sequence diagrams with a lot of great examples.