As noted above, major water changes will indeed bring your nitrate levels down in short order, The more important issue is to figure out why your nitrate levels are high. If you have been neglecting to do regular water changes, then it's pretty straight forward what you need to do to keep them down. If that's not the main cause of your high nitrate levels, then as nycbob said, more live rock and less feeding will help. Overfeeding may be the #1 cause of high nitrate levels-we all love to see out tank inhabitants get active and respond to us when we feed, but uneaten food and the fish poop produced by excessive feeding, will really shoot your nitrates through the roof, especially if you do not have enough biological filtration (means more LR, and an adequate sump filtration system and/or some macroalgae).