That is an excellent question and one that we see asked far too seldom on this forum. Campaign optimization is the primary function of a campaign manager. It is what you should be thinking about and doing on a daily basis.
Before I give you specific actions to take, let me explain the marketing principles behind campaign optimization. This will essentially be Marketing 101 in a nutshell.
One of the key concepts of marketing is market segmentation. By splitting your market audience into smaller segments you are better able to target your marketing efforts towards the needs of each segment.
The process of optimization is simply segmenting your campaign result data into small segments and adjusting your methods in one direction, or the other, to see if you can improve performance. As long as you see improvement in that direction you continue to make adjustments until performance degrades then goes back to the previous optimal adjustment. Now, with that basic concept in mind, you can find many ways to segment your audience and many ways to make adjustments.
First and foremost you should review your account structure. How you organize your campaigns and ad groups will make market segmentation easy, or difficult to analyze and adjust. It is virtually impossible to have a well-optimized campaign without a well-organized account structure. Do not overlook this step.