How does a GM use Paizo-style module flip-mats at the table without giving away the entire layout of the level? Take, for instance, Paizo's Emerald Spire Flip-Mat Mega-Pack… wouldn't putting one of these mats down spoil the sense of exploration for the players?
I use sheets of acrylic over top of paper maps and use butcher paper as 'fog of war' - I simply pull the butcher paper out as the PC's move forward. I place anti-slip cupboard liner under the maps so they don't slide too. Works like a charm.
The acrylic sheets are handy when you use dry-erase markers on top, too.
We usually draw dungeons onto a large battle map prior to the session. While it gives away a bit ahead of time, I always found it comparable to have a map of the city (or hiking trail) you visit - it's one thing that you know there is a large round place with a church building around the next corner and a completely other thing to end up on St. Peters Square in real life.
In a RPG session, the interesting thing is the description of what you find and that's usually not appearing on a map.
Other than that I've seen the use of a liberal amount of Post-Its (or plain paper) that gets moved/removed according to the players movement but I find this more of a hassle then it's worth.