Make sure that you are drinking plenty of plain water during the day. A minimum of 1.5 litres is a good idea.
Avoid all caffeine, including that found in fizzy drinks and hot chocolate. Drink calming teas such as lemon verbena in the evening.
If you wake up with palpitations, drink some more water, as this often calms them down.
If your life is quite stressful then you may benefit from taking a B vitamin supplement and/or trying a calming herb such as Avena sativa during the day.