There is always the time tested way of always riding with someone who wants to show off.  If you start noticing a flat, casually mention to your riding partner that you can change a tire in less than 3 minutes.  Your partner will undoubtledly say "That's slow, I can do it under 2 minutes!"  At this point, all of a sudden pretend to realize that you have a flat.  "Darn! What do you know, I have a flat...can you really change a flat under 2 minutes?  I can set my watch timer.  Care to prove yourself?"

Always worked....that or carry a cell phone.

But really you should know how to change a tire.  We found this video on YouTube courtesy of Mach3multisport.