Some tips for more effective white-boarding?

I am occasionally called upon impromptu to white-board (non-virtually) data flows, architecture diagrams etc., for both a technical and non-technical audience. Unfortunately my drawing skills (and print legibility) are terrible.

How can I become more effective at doing this? I am looking for tips on standard symbols and connectors to use, some standard ways of organizing and categorizing the information (e.g., swim lanes), etc.

What can I practice to become better at this? I want these visual presentations to be effective in communicating my ideas, and badly presented diagrams can make the ideas appear convoluted and inelegant, even when they are not.

White-boarding is a great tool. I do quite a bit of it myself, and I've found a couple of things to be very effective:

• Use a minimal set of symbols: Boxes, arrows, circles, and lines get you a long way. Prefer simple things to more advanced modeling techniques - everybody understands boxes and arrows.
• Think aloud while drawing to help the audience understand what you're drawing.
• Communicate with your audience. White-boarding is not one-way communication. If you are unsure whether a message got through or a drawing is understood, just ask.
• When the audience is sufficiently small, get people close to the board, and make pens readily available so people can draw with you. This allows for better visually aided communication, and an even more efficient white-boarding session.
• Take enough time to write and draw "neatly", but prefer a steady communication rate over perfect hand-writing. This is a hard trade-off which requires some practice, and practicing while keeping your writing and drawing understandable will increase both your writing and drawing speed.