[iJenny]

I do it randomly too. If its a close friend or someone we know well, its usually just their first name. If its someone we don't know very well or a person of importance (ie sunday school teacher, minister, etc) I say "Miss" or "Mr." followed by usually their first name. The only time I ever really do Mrs/Mr. Last Name is if its someone we've just met, an elderly person, or a school teacher/principal/etc.