Working out & Cardio - both will increase the speed of your metabolism. Working out especially, can boost your metabolism for 24 hours + because of how much energy it takes your body to rebuild your muscles after they've been broken down & all the nutrients it has to send towards that process.
That's the best way.
Other than that, try getting more protein in your diet. Your body has to burn calories to digest calories...even digesting food takes energy. Your body has a harder time digesting protein than it does carbs or fat...because of this it burns more calories when a higher % of your diet is made up of protein. Proteins also good for keeping you full, in general its very handy on a diet!
Try getting more active also. A person who stands up all day is going to be burning quite a few calories than someone who sits down for most of it. A lot of people with high metabolisms do these kind of things naturally.....mess around with things in their hands even while sitting down. Anything that causes activity will burn calories, even the small things. It all adds up in the end !