One thing I've learned is to ask my boss if everyone on the list is critical, or if there are some folks who can either send a proxy, or be brought up to speed at a later date.  

Sometimes, it causes Boss to stop and think about who he REALLY wants to meet and why.  Most of the time, at least one name is knocked off the list.  (Oddly enough, often the person who's dropped is the biggest pain to schedule so.... )