I've read that you can soak the planks in beer, wine, apple cider vinegar, or jack daniels. Another way is to use water with fresh herbs like rosemary, basil leaves or tyme.

My favorite is a plain cedar plank, garlic salt, and a light coating of brown sugar. This makes a glaze that compliments the cedar taste. When I made it up last time my buddies wife wouldn't eat it because she saw me put the brown sugar on and didn't want to eat "candied fish". Once we convinced her to try it, she polished off the plate!