There are two reasons a plastic bag works to reduce condensation:
1. It limits the amount of air (and therefore the moisture it can contain) around the camera.
2. It prevents warm, moist air from coming in contact with the cold camera.
Warm air can hold more water vapor than cold air without condensing. When warm air is cooled, it's capacity to hold water in vapor form is reduced, and some of the vapor will condense to a liquid.
If your climate is cold, when going from inside to outside, make sure your gear is well-ventilated to remove warm, moist inside air from around your equipment before condensation has a chance to happen. When going from outside to inside, place your gear in plastic bags or some other sealed container so that everything is surrounded by the dry outside air until after it all has warmed to room temperature.
If your climate is hot and humid, reverse the procedure above--the cold dry air will be inside, and the warm moist air will be outside.