Help!

Hi, I'm starting to worry about the writing part of my prelim. Every essay I have written this year has only been worth 16 marks no matter how much I try to improve it. The interpretation should be ok-I got 35/50 for a timed interpretation we did in class-but the writing scares me! Have you got any advice on how to prepare well for this and what makes a good critical essay?
Thank you and I hope you had a good Christmas!

SS