In the New Task Screen or the Edit Screen directly under the Toolbar, you'll see a list of After Save Options options. Select the 'New Task' option from that list.

When you press the Save Button, you will be brought back to the New Task Screen so that you can enter your next task.

Tasks even remembers the parent task (if there was one) from the previous task to make it easy for you to quickly enter a bunch of sub-tasks.