We are doing a door decorating contest in our school for Drug Free Week too, and some of the ideas i've seen are....
1) A big mirror made out of tinfoil on the door with the caption
"See Me, I'm Drug Free"
2) Photographs of the students making scared faces with the caption
"Drugs are Scary" (this teacher used thought bubbles coming from
each child's picture with things they could've been thinking....
3) "Slam the Door on Drugs" with a door closing on a witch's face
4) A big ghost on the door with the kid's names written around it
with "Say Boo to Drugs"
Hope this helps