For both the whiteboard and code challenge as well as for answering technical questions during the interview practice will help you move from terrified to competent to comfortable. Before your interview review questions on the languages/frameworks ect you will be using. Get into the habit of doing daily code katas. These will help you with both whiteboard and coding challenges.
Code katas help you to build solid practices in a controlled environment. The idea here is to create a situation with a known problem space and use it to safely practice the same thing over and over again until the technique is fluid, solid, and relaxed.