This is sort of a tutorial for the novice outdoor flasher and hopefully a catalyst for some discussion of the subject.
I'm talking about "run & gun" type shooting with a hotshoe flash when you're not planning on a lot of subject cooperation. The points here will apply whether you're a hired event shooter, grabbing candids at a public event or everyday family snapshots. This is not about formal portraits or off-camera flash.
And before you ask... no, you don't use a "diffuser" on your flash outdoors. They won't do you any good and they waste power. Direct flash is the only viable option.
One of the first decisions you need to make is whether or not to use High Speed Sync (FP Flash). This mode allows fast shutter speeds and therefore large apertures. It's a nifty tool for when you want to blur the background in bright conditions. But it is less efficient, cuts your range in half and leads to longer recycle times and shorter battery life. The decisions is yours, but be mindful of the limitations and keep an eye on your flash unit's distance scale.
I highly recommend using Manual exposure mode on your camera. It's important to take control of the camera and make deliberate decisions regarding exposure. If you don't use high speed sync then you'll want to set your shutter at X-sync speed and stop down the aperture to get the ambient exposure you want.
Now a word about Flash Exposure Compensation (FEC): The best FEC setting depends on the camera, your subject, and the reason you're using flash to begin with. I have used FEC settings from -2 to +2 outdoors. Don't let someone tell you where to set your FEC. It's just something that only experience can teach you.
So, let's go through some examples of situations where flash can be used to improve an outdoor shot.
#1 - Harsh shadows on a sunny day. Flash will reduce the contrast from sunlit to shaded part of an image. This improves faces dramatically. Properly implemented, you will get a natural looking image that still has hard-edged shadows from the sun but has decent exposure of shaded areas.
#2 - Backlit subject. Putting the sun behind your subject (if you're lucky enough to have the luxury) can make some real nice portraits with fill flash. The sun makes a nice hair light. This girl would be terribly underexposed without flash.
#3 - Shaded subject, sunny background. Without flash, you have the option of underexposing your subject or blowing out the background completely. Flash allows you to meter for the background and properly expose your subject.
#4 - Light up the face under the hat. Just be sure to rotate the camera to vertical orientation or you'll make a nasty shadow under the hat brim. 'Nuff said.
#5 - "Just because." Even on an overcast day, a bit of flash will make your subject "pop" by exposing her just a bit brighter and providing catchlights.