What's the best way to store that 1/2 gallon of paint left over after
a project?

I don't think I've ever opened a used can that's a few months old
without finding rust along the inside lip or a film of dry paint on
top. I am very careful about cleaning the lid and the seam where the
lid seals and making sure the can is closed tight - rubber mallet
tight.

This morning I had to strain a 1/2 gallon of Kitchen & Bath paint
because of the rust that fell in when I opened the can.