Try to capture the emotion and hold it within yourself. Then imagine yourself putting that emotion on something and watching it drift away.

Feelings are often driven by our physiological responses to a stimuli. Capturing the emotion is essentially "owning" the feeling and validating the feeling within you. Yes, even if it depression or mania it is still a very real emotion and should not be discounted. Holding the emotion in silence gives you time to understand and know what you're dealing with. Taking the emotion and releasing it shows you have some control.

Essentially what you're doing is desensitizing yourself to that feeling. It doesn't make it any less powerful, but you're able to rationalize through it.

And two other hints:
1. When you feel yourself getting upset, start counting and focus on counting to x. (I used to count to 10 and then take a deep breath)
2. Don't act on how you're feeling right away if you're unsure of yourself. Usually if you go to sleep and wake up the next day you should be feeling okay. Practicing this will help remind you that feelings are temporary. Even though you read this and you may comprehend it, you HAVE to do it. You may rationally know I am "telling the truth", but your body thinks I'm "lying".

For depressions, remember to give yourself small rewards for tasks completed. If taking a shower is a task for you, and you do it....treat yourself to milk and cookies! (example)