The last thing you want to do is drop your camera and inadvertently damage nearby coral. Avoid taking photos at the beginning of your dive. Divers tend to be most clumsy during the first 10 minutes of a dive, so take time to get acclimated before you start taking pictures. Remember to act as a guest, not a customer. Descend over sand patches rather than directly over the reef to get that next shot.
Sand is much more forgiving than the reef.
Don't get too close to the bottom. When taking pictures, remain at least one arm's length away from the bottom at all times. Be aware of your body position. Stop swimming and tuck your legs if you feel yourself hit the reef. Use your hands only for holding your camera and maneuvering.