What do you focus on, though?

What seems to be tripping me up is knowing what to focus on.

Do I keep my focus on one body part all day long?
Do I switch it depending on the activity that I'm doing at the moment?
If you do keep changing your focus, how do you constantly remember to change the focus of where you're feeling gravity if you find yourself being called to do various acrivities that you weren't mentally preparing yourself for?