Tie a burlap sack over the scarecrow's head if the head is only made of straw. Draw a face on with a permanent marker. Consider using the eye and mouth shapes similar to a jack-o'-lantern to enhance the fall motif.
Find an old hat for your scarecrow. Straw hats, baseball caps or even a cowboy hat would work well. Tuck autumn leaves into the brim for a fall look. Alternatively, tuck the leaves' stems under the hat with the leaves hanging down to act as the scarecrow's hair.
Put a flannel shirt on your scarecrow. Flannel is reminiscent of fall's cooler weather. Pin the shirt in the back so it doesn't hang too loosely in front. Tie off the sleeves with twine to prevent the scarecrow from coming apart at the arms. Tuck more autumn leaves in the shirt's front pockets.
Outfit the scarecrow with pants. Use old jeans or overalls. Use any size, but remember that smaller sizes will better fit your scarecrow, depending on its size. Pin the pants in back to get a proper fit, and tie off the cuffs with twine.
Tie a fall-themed bandana -- such as a pattern of pumpkins or colored leaves -- around the scarecrow's neck.
Place fall-themed props with the scarecrow. Tie the handle of a small, plastic jack-o'-lantern at the bottom of one arm and a small rake at the bottom of another to make it seem like the scarecrow is holding these items.