Preventing over and under exposure:
1) Pay attention to the picture on your rear LCD screen
2) Look at the histogram and make sure it is not bunching too far to the left or right.
3) Adjust exposure through aperture, shutter speed or ISO to compensate.
4) If shooting in very contrasty conditions decide whether you want the light or dark to show detail.
5) Use the correct auto-exposure metering mode, evaluative, centre weighted or spot.
6) Do you need to use flash, either as your main light source or as fill-in?
7) Position your subjects with their back to the sun.