Off-camera flash is arguably the best way to improve your photography.The general idea is to make use of small portable strobes to light your subject, much like studio lighting, but without the limitations of power sources or locations. If you don't own a flash yet, please read last week's article first.
Update: This article was published in June 2011. For current information, please read our latest guide to off-camera flash accessories, published in March 2014.
Why off-camera?
Since the dawn of the first photographic studios equipped with flashes, photographers have experimented with removing the flash from the camera. By being able to move the flash around freely, your ability to make use of light is vastly improved. Off-camera strobes can be used to make a photo look more natural, as if it has been lit with natural light instead of the telltale light from a strobe closer to the axis of the lens (on camera), or the strobes can be used to enhance the photograph otherwise - the old cliche of "your only limit is your creativity" most definitely applies here. Of course, making the best use of off-camera lighting is easier said than done - the same rules that count for studio lighting count here, no beginner photographer can walk into a studio on their first day and walk away with the best lighting possible - but practice makes perfect, and you'll enjoy shooting with off-camera flash so much that the great results will start rolling in sooner than you think.
How do I get the flashes to fire?
Update: We have some new triggers in stock!
If you have never experimented with off-camera flash, you might be wondering how to get the flashes to work with your camera. A flash needs to sync up with the curtains of a DSLR's shutter. If the flash is not synced up properly, you'll either have completely black frame, or a dark bar across the bottom of your image. Most cameras can sync at 1/250s these days, but 1/125s should work for most situations. (Alternatively, you can always turn your camera upside down...)
Luckily, there's a lot of methods to get the flashes to sync with your DSLR. You might not even need to buy any new equipment - if you own a fairly recent Nikon camera and a CLS compatible Nikon flash, or a Canon 7D/60D and a compatible Canon flash, you're already sorted. These cameras can use the built-in flash or infra-red transmitters to communicate with the flash and tell it when to fire. You also get to make use of through-the-lens (TTL) automatic modes, which can be useful to begin with, but will remove a bit of your control over the outcome of the photograph. Additional extra's, like the Canon ST-E2 transmitter accomplish the same thing, but extends the compatibility to cameras without built-in flashes. Infra-red communications have one big disadvantage though - by making use of light to send the signals, they can struggle outdoors or in situations where the camera doesn't have line of sight with the strobe. Some flashes, such as the Nikon SB-900 also have an optical sync, much like larger studio strobes. This allows you to use another flash on the camera to trigger the remote flash, the SB-900 will pick up the sudden increase in light and trigger itself in time to expose the photograph. Once again, this suffers from the same problems as infra-red, as the remote flash might not trigger in direct sunlight. (Most built-in flashes will fail to trigger a remote flash using this setup, as the TTL pre-flash will set off the remote flash before the exposure happens.)

What about positioning the lights?
Now that you've got your lights set up to fire remotely (mentally, at least), how do you go about making use of them? You'll need a set of light stands and "umbrella holders" to begin with. Umbrella holders allow you to attach a speedlight, which uses a hot shoe, to a light stand, which uses a standard spigot. The holders are specifically designed for this purpose, and offer you a range of adjustments and movement for the speedlight, as well as its namesake slot for attaching an umbrella light modification. Two stands, holders and umbrellas will get you started with a very decent off camera flash kit.

Sure. Speedlights make use of batteries, and as such you'll need a few sets of rechargeable AAs. This also means that they don't cycle as quickly as studio strobes - make sure to wait until each flash is charged before you take your next shot. Speedlights are also not as powerful as a full-size studio flash - about 4 times less. This means you need to double up flashes to overpower direct sunlight...
Select images © David Hobby.Please feel free to ask any questions in the comments!