Or simply calm down. The advice and techniques aimed to help someone calm down are usually a good idea, well intentioned and only partially useful. Not because they are wrong or improper but simply because most of us don't know how to do it - we don't know how to make the breathing or the counting work.
The goal if achieved is to get to a state which many authors and even religions talk about. Some ways that describe it are : clear mind, getting in touch with your inner self, being at peace, activating your thinking brain and so on. I think all of them describe the same condition which is generally useful not only when you need to calm down.
What is it and why is it so useful? I prefer to use a brain analogy to answer these questions. So a little background first. A part of your brain known as the pre-frontal cortex (PFC) is responsible for your executive functions - eg without it you would not be able to plan how to go to the shop or what to do next Saturday. Now the key about it is that it has very limited temporary storage - 4 or 5 bits of information and it takes a lot of energy to keep these bits there for long.
Overworking the PFC is a bad idea. Your brain becomes slow an you're likely to make bad decisions. In key or emotional moments the PFC will struggle to function properly. By quieting your mind you help the PFC mainly by making space for useful ideas to be brought to its attention. These ideas will usually be good enough to get you out of almost any situation.
All of the suggestions I mentioned at the start when executed properly are ways to make space in your PFC. For example with breathing what you need to do is focus exclusively on the breathing trying to get rid of any other thoughts. Breath in slowly then breath out trying to relax those parts of your body where you feel tension and all of the time focusing on your breathing and trying to avoid any other thoughts. ( see mindful breathing for more information)
So next time you need to calm down why not try taking a deep breath but using the technique described here and see if it makes a difference?