Whiteboards are my preferred method for this sort of thing. The building I work in is actually awesome for this; the safety shields (mol bio lab) at the end of everyone's desks double as whiteboards and the windows are write-on-able as well.

I also know a guy who used to have a piece of whiteboard paper glued the back of his laptop so he could close it and quickly sketch something out if he needed to, his desk is also covered in the stuff.