How do I keep the butter and sugar I'm trying to cream with hand mixer from flying out of the bowl?

Whenever you mix—whether with an electric stand or hand mixer or when you are simply tossing ingredients—try to do so in a bowl large enough to accommodate at least twice as much volume as you have. It leaves plenty of room within the bowl for the ingredients to really move around which makes mixing more efficient. And yes—your counters and floor should stay much cleaner!