Try it on paper or something first so you can get an idea of it more. Then just transfer what you've learnt to doing it on the shell. Don't try and do it all in one go, gradual shading as Jon has mentioned will work perfectly.
On your first pass you can make it quite loose, then the more passes you do the closer to the body you can bring the airbrush, also control the paint more. The natural effect you get from this will give a shadow that is closer to the area you are shading and softer the further away it gets, like a real shadow.
Or you could mask it into the design and then just mist over some transparent colour that is suitable before you put the design down, this will give a very graphical effect and although not the soft 'photo real' type shadow it will be very effective !
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