The preparation of your camera is as important as preparing your diving equipment and you should take as much time as you need to do that . If not doing so you will face problems like foggy lenses, out of power situations or in the worst case (also happened to me) a leakage.
After many trys we are preparing our camera in a dry, preferably cold environment. If you don’t have this you should consider placing the camera in the housing in the early morning or the day before.
When doing so you are also checking all O-rings and cleaning them if necessary. This will also be done with the used lenses and batteries (preferably rechargeable ones). There is nothing worse than a shut down of your camera or lights because of empty batteries while filming a almost perfect scene. You should think about buying the rechargeable batteries for your camera with the highest available capacity. In dirty and wet environments you can then do two dives without opening your housing and this will minimize the risk that dirt or sand will be placed on the sealing.
You should pay the same Attention to your torches for the same reasons and with the same procedures explained above.
Secret tip: housing made of aluminum or metal are better than the ones made of polycarbonat / plastic when it comes to the Problem of foggy lenses. In Addition I am using a special hose and the extremely dry and cold air from my bottle to fill the housing with it before I am closing the housing.