When pruning, cut back, at the first outside shoot on each branch, do not cut back from an inside shoot. Cut any straggly branches back the plant will then grow stronger.
Shrubs that flower in spring--such as forsythia or lilac, should be pruned shortly after they bloom. If you prune any later, you will be cutting off the next year's blooms. Shrubs that bloom later can be pruned anytime.
In general, all flowering bushes should be pruned soon after the flowers die. You can prune back about 1/3 of the stems, but keep the appearance of the bush in mind.If you wait too long, the bush will set next years flowers and you will cut those off.