Generally the light set up for high key is the model and the background each have their own lights. I set up the background (usually) first and get it set. Usually I set it at about 1 stop over what I plan to shoot the model at. If I'm using f8.0 for the model, I'll set the backdrop at about f11. Obviously this needs two lights minimum and metered separately.
The barn doors is what you want (or something similar) to keep the background light from spilling onto the model. Also as said before, it helps to move the model further away from the background.
From looking at the shot you posted, it looks to me more like lens flare; that is some light (probably the background light) is hitting the lens. I use the barn doors and a lens flare guard and sometimes a gobo (piece of cardboard or foam held between the light and the camera to keep the light off the camera).
Don't go too much over the one-stop difference or you're liable to get lens flare from the background itself.
Hope this helps, keep shooting!
(I would have put an example up, but I don't have any in my portfolio at present, sorry...)