Normally, I am comfortable when I have to explain a piece of mathematics to a group of people using pen and papers as supports. However, I find it quite difficult to give small talks and explanations using the blackboard as a support.

This is mainly because the blackboard is huge, my handwriting has to be quite big (to let students see from their seats), and I have to stand at, say, 50cm from the blackboard: as a result, I find it extremely difficult to have a complete picture of what I'm writing (to make one example, sometimes I cannot even keep track of the letters I've used to denote variables) and to follow my own reasoning (whereas, on a piece of paper, I would have everything under better control).

What do professors do to avoid this kind of loss of bearing when using such huge blackboards to give extended lectures? Could you give me some advice based on your experience as students and educators?