I tend to set up maps on new pages then put all the creatures or extra bits I want on this map. Anything that would not be visible to the players initiallly I move into the GM level. If it's a dungeon or dark room I use the fog of war setting to completely blackout the map to the players then expose the bits the players would see when they enter it. Also I put the player's characters on the map where they would first appear. Makes it easier to work out what they would see and also makes the transition to the new map easy. Then as the game gets to this bit move the player marker to the new map and it appears to them. Hope this helps.
