Consider the context
Design the experience with use in mind: one hand for multi-tasking, two hands for speedy typing, and landscape for watching movies.
People hold their smartphones and tablets in different ways. Understanding the different ways allows you to design interfaces that help people be more efficient.
Smartphones are used primarily in portrait view. In our busy lives, we often use smartphones with only one hand. Make sure that people can carry out the most common actions in your application with one hand, preferably without changing the grip. On all-touch smartphones, be careful when planning the layout for your application and strive to place the most common actions on the bottom two-thirds of the screen. For example, a list of search results grows from the bottom so that people can easily reach the top hit with their thumb.
On BlackBerry smartphones with a physical keyboard, you can consider using shortcut keys to give users direct access to common actions for a specific screen. For example, allow users to press "T" to move to the top, "C" to compose a message, "S" to search, "I" to zoom in, and "O" to zoom out.
When people are willing and able to use two hands, your interface should include features that make common tasks quicker with two hands. For example, you could include a context menu in your application to make triaging email much faster.