When looking at a High Key picture, you will probably notice two things right away. (Other than the happy-happy-joy-joy mood of the picture) The first thing is that the picture is bright. Yes - to create a high key image you need to set your exposure levels to high values. You will want to watch out, though not to over expose.
The other noticeable feature of High Key images is the lack of contrast. In addition for the tone being bright, you will notice that it is almost even across the scene. This is achieved by carefully setting the lighting of the picture.
A third feature, that need closer attention to notice is the lack of shadows in the picture. The shadows cast by the model (or subject) are suppressed by lighting in the scene.
the above bit was pinched off the web but I felt it appropriate to your question in terms of firstly getting the image right in camera. the second stage being to process the image to pull the best from your shot..this part is somewhat subject to personal preferences in editing...I personally would take the shortest route possible... by this I mean if theres a photoshop plugin that dos it for me or gets me close to what im after il use it! the likes of nic software, onone software tc all have presets that will get you pretty close without you having to spend hours on an image.
hope this helps a little
cheers Andy