You may have to do two things since you are wanting the expression to be saved, because I haven't tested this out...
Once you set everything up, the label placement properties, expressions, font size/colour etc..., from the Labels tab in that layers Properties window, click the Label Styles button and then click Save.
That will save the label style with its current placement properties and everything you set up and can be used on another layer when you need it, but as for the expression, I am not so sure this will save that. From the label expression window, you may be stuck with using the Save/Load buttons to constantly import your previous expression for use again.
Sorry I can't answer the 'default' setting part of the question.