First of all, with beech and hornbeam do not cut back the leader, ie the tallest upward growing branch, until the hedge has reached about a foot above the height you want. Generally beech stops growing upwards properly when the leader is removed. Secondly to make the plants bush out lightly prune the ends of any branches which come out forwards or backwards. Thirdly any branches growing sideways may be woven into the trees along side. Have fun!