The only two proven methods that will work EVERY time if done correctly are Copper and Hyposalinity. I prefer hyposalinity, but you must have a refractometer to do it, hydrometers aren't accurate enough. Either method requires treatment in a q-tank. Also, like previously mentioned, the main DT needs to remain fishless for 6 weeks to guarantee that all of the ich parasites within the tank are dead.
Here's a couple links on ich treatment methods.
ATJ's Marine Aquarium Site - Reference - Marine "Ich" Marine Ich/Cryptocaryon irritans - A Discussion of this Parasite and the Treatment Options Available, Part I by Steven Pro - Reefkeeping.com