In Friday's speaking lesson I described a funny thing that happened to me. I found it really easy to write my description because I told a true story. I wasn't worrying about grammar or vocabulary; I just told my story in a natural way.
The key to a high speaking score isn't your use of 'complex' grammar structures, big words or idiomatic phrases. The key is to speak as naturally as possible, and real examples or stories help you to do this.