This may seem like a trivial and/or simple principal, but I feel it is key to learn how to hold your camera and make it a habit.
Early in our relationship, my wife Abby learned and adopted the correct posture for using a digital SLR, and has used it ever since, and I think it really makes a difference in her shooting.
The correct way to hold an SLR is to place the lens in your left hand so that the base of the camera sits on the heel of your hand. You fingers should then naturally wrap around the zoom and focus rings of the lens. The right hand goes around the grip of the camera’s right side, with the right index finger on the shutter release. This yields a position from which you can operate all of the critical controls of the camera without removing your hands. It also has the advantage of creating a natural “tripod” effect, with your body acting to stabilize the whole camera. To shoot vertically, the left hand stays where it is, and the right hand moves the camera to the vertical position without releasing from the grip.
Cameras are designed to work this way. I am always amazed when I see people struggling to reinvent this very obvious wheel, and I know they would do better if they learned to hold their cameras the right way.