Take a quick inventory of the things which most occupy your mind. You might find that you've been stuck on the same set of problems, hopes and fears for weeks at a time, perhaps longer. Certain thoughts are heavier than others. Not only are they heavy, but they loop back, over and over, keeping you from being calm otherwise and from completing tasks you otherwise would.
To master the landscape of your thoughts, create a dualistic way of thinking: pay attention to what is crossing your mind. Be aware of what has come into your sights, even if that at first just means you saying mechanically to yourself, 'I'm aware of this thought.' This sounds simple, and it is to a certain extent, but you'll find that you start doing it in layers. Not only will you become aware of your thoughts, but you'll notice that you start being aware of the value judgements you assign things and instances. Doing this helps you approach everything, no matter what it is, from a neutral position which allows you to come to a clear conclusion about the best mode of action no matter what your feelings are on the matter.
You'll still get angry; it just won't control you. Your anger scatters when you stare it in the face. Same with fear, insatiable longing, discomfort and anxiety. You don't eliminate them completely, but they are not so eager to consume you. They become more natural; passing humors you can let work through you like any other experience.
If you pay attention to yourself enough, you'll find yourself unable to help but to live in the moment. It will start to become more and more natural.
Soon, you'll be able to see certain emotions and thoughts coming from a long way away, sort of like clouds or high flying planes. As they pass, you will have ascertained more details about them, including the likelihood of their frequency, what kinds of situations cause them and, most importantly, if they're thoughts that are even worth having. Some thoughts need to be nourished while others simply need room to say what they need to you before they dissipate. Over time, it will help you focus and give you a rich inner life occupied only by those things which are edifying to you.
It can be paired with other practices too. Some would rather pray. That can do something similar, but on the other hand, how will you know what to pray if you can't think straight? Pairing prayer with mindfulness helps tremendously because one can focus on what the prayer is really trying to accomplish or do, rather than being the sort of last minute squeak of alarm we utter when we almost get into a car wreck.
It's a simple method. It's not always easy to execute, but it's worth it. Though simple, it can lead to a wide variety of benefits over time that you never would have thought possible.