My tip would be to keep a pad of paper on your desk and jot out ideas before implementing them.
Use the pad of paper as much as possible and then you get practice in converting thoughts into pictograms. Because you draw for only for yourself there is no fear of failure.
Its actually quite natural -- we used to live in caves and we drew pictures of the animals we would hunt.
Edit: I used to be terrible with whiteboards.
Sounds to me that you need to slow down. Respect yourself enough to believe that your colleagues' time will be well-spent if they give you their attention.
Could be too that you need to brush up on your drawing/diagramming skills...
Sounds like you may have two possible issues and I can't tell which one the question is about.
1. You have difficulty explaining or articulating your ideas in general 2. You have difficulty translating your thoughts into physical interaction on the whiteboard.
For the second one, I just think of the whiteboard as a place to bookmark or store the stack of a discussion. Usually the rest flows from that idea for me.