Think visually. Any action you write will be depicted in pictures. Be descriptive. Remember, an artist has to interpret what you have written and draw from that.
Work out the story ahead of time. Know the ending. This will help to keep the story focused. Unlike a novel or short story, you have no room to expand or to go off in different directions in a comic book script.
Keep the story simple and basic. Less is more as far as comic books are concerned. Write a fast-paced story. Use dialogue only as needed to get a point across.
Focus on the characters instead of plotting. The characters are what readers will identify with. Create something to like about each character, heroes and villains alike. Make them interesting.
Study writers working in the field now. Read comic books being issued by different publishers to see which publisher fits best with your script. Check out script guidelines for different comic book publishers.