Ergonomics matter: consider how your app feels in the hand.
Put primary controls in the thumb's "hot zone."
Forty-four is the magic number. Make tap areas at least 44 pixels, and design to a 44-pixel rhythm.
Be generous with space and don't crowd your design.
Feature primary content at the top, controls at the bottom.
Keep the main controls within easy reach, and avoid scrolling where practical.
Reduce interface chrome by dispatching power tools to secondary views with secret panels and hidden doors.