But before talking about how to be relaxed in mind and body throughout the day, the first question I must ask you is whether you feel relaxed in mind and body when you are meditating. If not, then that is something that you must approach first before addressing how you feel throughout the day. See my post, "Meditation - Coming Home, Releasing, and Being at Peace."
For most people, though, the answer to that question will hopefully be, "yes." But as we go through our day, we are bombarded by all sorts of experiences which are a catalyst for our emotions. If you are at peace and relaxed, you will still experience your emotions, but you will not react to them. If you are not at peace and relaxed, you will react to them and have a negative spiritual experience; you will suffer.
There are 2 exercises you can do periodically throughout the day to keep you centered on your practice and be relaxed, both of which are very simple and short. The first involves the use of the Korean word, "ma-um," which means heart and mind. The exercise is to say, "Breathing in, saying 'ma,' I relax my mind. Breathing out saying, 'um,' I relax my body." Breathe in through your nostrils, expanding your abdomen. Breathe out again through your nostril, and somewhat sharply, drop your shoulders. Do this 3 times.
The second is a deep breathing exercise. Breathe in for 4 counts, 4 seconds, expanding your abdomen as far as possible. Breathe out for 8 counts, 8 seconds letting your abdomen completely empty of air. To increase the effectiveness of this exercise, it is helpful to hold the breath for 4 counts, 4 seconds, after you breathe in before you breathe out.
One of our greatest challenges is staying calm and relaxed throughout the day and responding to our experiences from our heart not our mind, our emotions. That requires being present throughout the day and being mindful, neither of which you can do unless you are in a calm, peaceful state. Practicing these short exercises helps you maintain that state throughout the day.