The only straightforward solution I've heard before is to use an opaque layer in photoshop over the actual map, and slowly erase it as they explore. I don't own photoshop, sadly, so that's not a great option. (Can anything similar be done in MS Paint or GIMP, perhaps?)
I don't need a technical solution, in fact - if there's a good physical way to solve the problem, that's fine too (I've used Sticky notes obscuring a hand-drawn map before, but it's kinda tedious and clunky).
I can always resort to having a personal hand-drawn copy, and drawing new chunks out on the battlemat as they find them, but something quicker would make my job a lot easier, since I'm always busy in the middle of a session.