Breathing to relax the body needs to be practiced so it becomes your normal way of breathing. The best times to do this are morning upon waking and evening before sleep. This technique will help you drift of to sleep too.
Simply breathe into your stomach for the count of 4-5 and breathe out for the count of 6-7. The idea is that you make your out breath longer than your in breath. The out breath is the relaxation breath, you want your body to be in relaxation more often than it is in alert mode which is the in breath.
Practice this 10-15 minutes, 2 times per day for up to 3 weeks and you should notice that your breathing pattern will change and you will be using this breath pattern naturally during your day. You can also insert this breathing exercise into your meditation time.