This is an easy technique I read about a long time ago, and it works really well. I use it a lot when shooting weddings and portraits.

Outdoor portraits can often end up with a washed-out and boring sky when you set your camera to correctly expose the persons face. But if you set the exposure to capture the sky perfectly, you end up with a very dark or even silhouetted person. We all know you can use fill flash outdoors to correct this, but even then the exposure can be a bit unpredictable if left up to the camera.

The technique I like to use is:

• take a meter reading from the sky
  
• set your camera to Manual mode
  
• set the aperture according to the depth of field you want
  
• set your shutter speed to correctly expose the sky (based on the earlier reading)
  
• turn your flash on and set it to E-TTL (or whatever the equivalent is on non-Canon cameras)
  

You may need to tweak your aperture setting (or ISO, if shooting digital) if the required shutter speed is faster than your flash sync speed.

The manual exposure gives perfect exposure for the sky, whether its a deep blue with white fluffy clouds, a sunset, a looming storm, or whatever. The flash will light your subject perfectly, giving a nicely balanced overall picture. You might need to experiment a little to determine whether you need to use flash exposure compensation - I get good results from my EOS 20D/580ex combo without any compensation.

One problem that might occur is that your on-camera flash can make the subject look a bit flat, giving the photo a fake overall look. A diffuser like the Lumiquest Big Bounce can soften the edges of shadows and make your lighting look a lot more natural, especially if you can combine it with an off-camera flash cord. Even better would be a mobile studio light with a softbox or umbrella. Just make sure you get the lighting direction from the flash right so that it doesnt contradict the direction of the lighting in the background - that can look awful!