Flash is a touchy subject for many photographers, who either degrade it as amateurish - or write it off entirely as a technique that can`t be used outdoors. Here's a guide to using flash outdoors, including the "brute force" method, and the "fill light" technique, as well as some general tips.
Using Flash Outdoors
The use of the flash occupies an odd place in photography. Many photographers abhor it as an amateurish technique for creating light in dark places, and even those photographers who do use flash find it useful only in very specific circumstances. Yet flash can be a powerful tool when used correctly—even when used in such traditionally non-flash environments as outdoors. Here's an introductory guide on how to use the flash technique effectively outdoors.
Before You Begin
The light produced by flash is bright and intense; before you take your picture, consider carefully whether using flash is really appropriate for your circumstance. If you're photographing light sensitive objects (or people) abstaining might be a better idea; adjust ISO settings and utilize other techniques to replicate the same effects.
Also, make sure that you correctly adjust ISO, exposure and other options on your camera to account for the additional light. This might be difficult at first, so taking your camera out for a few shoots just to see how strong your particular flash is, is probably a good idea so that you know how much you'll have to adjust your settings when the time comes to use flash.
Some cameras and/or flashes will have something called ETTL, which essentially will make many of those adjustments for you. However, these will often have as many difficulties as adjusting the settings for yourself. It is best, as always, to experiment with such.
Without further ado, the two main outdoor flash techniques:
Flash As "Fill Light"
It doesn't have to be dark out to use the flash. Flash can be used with some subtlety to lift shadows, almost imperceptibly, to create a better balanced photo with regards to light. So, this fill flash technique is best for one where there is a more extreme contrast between light and dark. This technique requires considerable delicacy and a lot of practice to master. The flash will have to be set as low as possible, yet the exposure and other settings should still be set for the surrounding ambient light. It is usually best that the image is shot straight on. Usually, you will have to use it on the “force flash" mode, as the camera will probably not recognize the lighting as one that automatically requires flash. You might even want to use a diffuser of some sort with your flash to create a softer, more ambient effect so that the use of flash is all the more imperceptible, especially if the light from it is particularly harsh and you can't tone it down much.
Check out this article for some beautiful examples of the fill light technique.
Flash As "Brute Force"
Sometimes, however... it really just is dark outside, and you just need some lighting to get an image, any image at all to show up. However, just pointing your camera and flashing will create flat, dimensionless photos. If at all possible, try to bounce the flash off of something—the wall of a nearby building, a canopy, anything. Failing that, you can try for something a little more crude but not ineffective, such as redirecting or curving the light of the flash with a piece of paper or even your hand.
Keep in mind that the flash will only work at short distances. So, it is virtually impossible to get both the foreground and the background to be properly lit if using flash outdoors at night. A little creativity with settings might be required—placing the subject in front of a bush, for instance, so that an infinity of a dark forest doesn't create a black void behind your subject.
Of course, these same techniques may also be applied to similar lighting situations as your typical outdoors, for example in large indoor settings. Be sure to experiment to see what works and what doesn't—all it will cost you are a few flashes of light. Let your camera be your guide.