Tips posted by hyberneat
Shooting portraits on a sunny day
If you're taking pictures of people on a sunny day, always use the flash, especially in the middle of the day, when the sun light is coming from above.
The flash will usually not be strong enough to make a big difference, but it will help to reduce the shadows from eyebrows and nose, making the portrait look much more natural.
Never use flash when photographing through glass
If you ever want to take a picture of something behind a glass window (or a fish inside aquarium) - never use your flash!
The flash will light the glass and you will probably get just a big white spot covering exactly what you were trying to take picture of! Instead - disable the flash, but photograph with high ISO settings to compensate for the low light conditions.