The best alternative is to turn the fluorescent lights off. The typical lenses that cover fluorescent tubes are designed to spread light uniformly throughout the room, something not desired when doing presentations. These lenses are designed to throw the light straight down, and as such significantly less light reaches the projection screen. If you do not have separate light switch banks, you will need to twist one tube in each set of fluorescent lights to turn off the set. Our experience is that building maintenance will do this for free if you ask politely.
Dimmer switches work fine.
Whenever possible turn off the lights at the front of the room. These can be directed to throw a lot of light towards the back of the room and almost none onto the screen. The worst situation for any presenter is a room with a lot of windows. If possible, position the screen against the window wall so direct sunlight will not reach the image. In a temporary room, almost anything taped to the windows will help.