whats the best way to get a 'High key' studio image?

Answer 1

To add to perkie's answer, I will also light the subject in the same manner as the background, and over expose by a half to whole stop, obviating subtle detail in the highlight areas.

High key = bright, very bright, subject, background, minimal shadow area's. Most of the data in the histogram will be in the middle to right hand side.

Look for an answer from "ace", as fashion/ model shoots are his bread and butter. He will probably have more to add.

Answer 2

Use a white backdrop like the Savage Super White seamless paper. Light the background with it's own light or lights. Meter the subject area with a different light or lights but keep the reading the same as the background for a smooth white tone front and back. Years ago I was told to "blow out" the background but I eventually found out that metering the same created a far smoother transition on the paper.

EDITED TO ADD: here is a sample, a poor scan of a film shot, taken when my daughters were young. They are now 20 & 23, so that gives you an idea how old this picture is.
http://farm1.static.flickr.com/173/431857418_1c24881adc_b.jpg