Thanks Michele, I'm glad you found it interesting. Actually I'm just glad you made it through the post!
I'm really hoping that we both mean something slightly different by "alla prima." Please don't tell me you do those beautiful, highly finished portraits in one sitting, or I'm about ready to give up altogether. Perhaps you just don't sleep for days on end until you've finished?
I'm guessing that you are referring to painting wet into wet - pretty much the same thing I guess in terms of technique.
One thing I forgot to mention is that I have a 'no white paint in shadows' rule at the moment, although I confess I do break it sometimes. I find that white is too opaque and spoils the translucency, if I need to lighten a shadow I wipe off some paint with my finger to let more ground show through.