One quick and easy way of making any application or website more usable is to identify primary actions and secondary actions. Primary actions are the most frequently used that guide the user along an intended path to complete a task. Secondary action, while less used, offer the user ways of exiting / Modifying / restarting / Lableing /etc. a particular task. An example that all of us are familiar with are log in screens. Log in screens may consist of two text fields, one for your username and password, a log in button and (depending on the application or website) a register link, forgot password link, cancel button, etc. In this case, can you identify the primary and secondary actions? If you said the primary action is the log in button, you would be right! The frequency in which the log in button is used is far greater than the others. It may not be something you often think about, but identifying the primary actions of the user and drawing more attention to that action can significantly improve the usability of your application.
Here is an example from Gmail (Send being the Primary Action):
And a few from log in screens on the Iphone:
As you can see, color, shape and opacity can all be used to differentiate primary actions from secondary. Now, go back to that application you have been working on for the past few months. Do you see primary actions that look exactly like secondary actions? Now you know what to do!