As well as varying the length of your sentences you should try to vary their construction.
If you look back to your work in Year 7, you will probably find that in your stories you had sections like the following:
"He walked carefully into the narrow room. Then he saw a picture above the fireplace. He wondered who was in the picture. He walked over toit. He was sure he had seen that face before. Then he looked at the grey hair and the cruel eyes. He didn't know where he had seen them. Then he remembered that he had once met a friend of his father's..."
Try listing the elements of this story that make it repetitive.