I have worked with printed battlemaps before (for interior gameplay) and have often used post-it notes to cover the hidden areas.

I found something I want to try next game I DM (depending on the price...)

http://www.3m.com/intl/ca/english/ce..._features.html

Post-it paper in 8.5x11 sheets! I figure I can print each room on a sheet and stick them down as they go into them... And for things like hidden doors and traps I can just have those printed out as separate overlays to stick on top as they are discovered!

-Rob A>