Here is a tip: Don't let your prototype look too polished. Otherwise, the dear leader will think it is closer to completion or easier to implement than it really is and give you an earlier deadline. I would suggest hand drawings are best for UI prototypes in this respect. The closer you get to something that actually looks like operational software, the greater chance you have of some executive reeling in your deadline or some sales idiot promising a customer delivery next month. More mature companies have more discipline in this regard, you'll have to figure out where the line is in your specific company.
I read a paper a while back that observed the rougher (less complete) the prototype looked, the better feedback it got. Completed UIs get feedback such as "change this button text/color" or "move this button over there" while hand drawings got feedback like "I really need the system to be able to do X" or "this creen doesn't fit with the way i like to do my work".
I'll post it here if i can find it again.