From my personal experiences with GCSE Spanish, there are a few aspects to avoid and key factors to include.
So avoid:
Cliches "que sera sera" for example
Basic sentence structure "el edificio es feo." ""Me gusta educacion fisica"
Long, boring paragraphs. Keep it short and and concise
Remember to include
idioms - Collins does a great idioms book
A range of sentence structure ie "me gusta las clases de ingles, porque el profesor lo hace interesante, asi es muy emocionante"
Use a lot of opinions also, and use a thesaurus for better, more sophisticated language