Privet is much like grass in the sense that it always grows as long as the temperature is above 9 degrees celcius. Trimming the hedge will help keep it in shape and also make the hedge dense, rather than scraggy and untidy.
If your privet hedge has become unruly the first steps you should take are as follows;
~ Choose the height you wish to cut it back to. Do not be afraid of damaging the plant; privet is a very hardy plant and can tolerate strong pruning.
~ Once you have trimmed the hedge to the required size and shape, place a good helping of manure, or similar plant food like bonemeal, etc, on the soil underneath the hedge.
~ Once the privet starts to grow again, keep it in shape by giving it a light trim every 2 months or thereabouts (be careful not to disturb any nesting birds in amongst the hedge - especially around late spring to summer time). Giving the hedge a light trim should not affect wildlife but do avoid heavy pruning around this time.
Other points to remember; although privet is seen as an evergreen, leaf loss can occur under heavy pruning or harsh cold weather. It should recover without any problems though.
If you place privet into containers for topiary specimens, remember to feed it well as privet are nutrient hungry plants.