The first thing to do is try to calm down and take a break. Take 10-15 slow deep breaths, focusing on opportunities in the respiration process. Sometimes this is sufficient in order not to cry.
Try to "switch" attention. For example, remember what is written in your passport – all pages (surname-name-patronymic, date of issue, address of residence, and so on), count buttons on clothes of the interlocutor or stripes on the carpet, repeat the multiplication table for seven.
The most common reason for the "crying" is a strong, uncontrollable emotions. And one of the ways to keep yourself from crying is to "switch" from mental pain to physical. To cut off his ear like van Gogh, of course, extra – hard enough to pinch myself or bite the tongue or lips. It's a pretty effective way to hold back the tears and to bring itself to life.
If the cause of tears is resentment against a specific person, try to mentally redirect the emotions that you are experiencing, with themselves on it. For example, if the boss neither gave you undeserved dressing – don't feel sorry for yourself. Better be angry with her... or find what's to feel sorry for her (and my nerves she can't do anything, and climax soon, and the bad haircut). And can mentally laugh at the offender – to dress in a clown costume, transform into a crow or a frog... anything – just gone is the feeling that this man is a threat to you personally.
Not to burst into tears immediately after a conversation, drink a glass of water or hot tea slowly in small SIPS), and at the first opportunity, wash with cool water or at least wipe it with a damp face cloth. And immediately take any business not directly related to the fact that I almost made you cry – it will help you to fix the result.