Few/Fewer: for stuff you can count
How many hours of sleep did I get? Too few.
The Bulls have won fewer games since Michael Jordan’s injury.
Less: for stuff you can’t count
He needs less practice and more playing time.
“Practice” is a concept, and you can’t count it. You can count individual practices, of course – “I went to three practices this week.”
We should emphasize our common interests more and our differences less.
You might be tempted to say, “But wait! We can count our differences!” In this case, though, “less” refers to how much we should emphasize something, (“Let’s place less emphasis on X”) not the differences we’re emphasizing.