From spring onwards, once they start growing, water them a lot too. But dont do little and often as then the roots will stay at a shallow depth. Give them a good soak. Especially in summer.
Don't trim the top of yew plants until they have reached the desired height. In ideal conditions they can grow up to 18" (45cm) a year, but after you clip the tops they will only grow 2" (5 cm) a year in height.