Teaching Common Sense

What is the best way to teach a kid common sense?

I don't think you can teach common sense, I think people either have it or they don't.  I say this because of the number of adults who clearly have none... common sense is not common.

I will say this, though- if you're asking because of the bahavior of a middle school or high school aged kid, it's most likely just a part of their development-stage, as their brains really don't mature until they're in their 20's, and many of the connections aren't fully connected yet, so they do stupid things and don't know why, or think stupid things and don't understand that it's nuts.  That they'll grow out of. 

I think a good indicator of whether your kid will have common sense is to consider what they were like before puberty- if they had a fair amount of age-appropriate common sense before puberty, then their brain is probably just checked out while it continues growing, and their common sense will once again prevail.  If they've never shown signs of having it, well, then, I hope you have alot of patience...