A good skimmer will remove a lot of organic waste from your tank before it can break down into nitrate. How old is your tank and what is your water change schedule?
This article recommends a few additional ways of reducing nitrate such as growing macro algae, employing a deep sand bed, and removing filtration materials that facilitate the nitrogen cycle.
http://www.reefkeeping.com/issues/2007-09/rhf/index.php