The most common failing in essays, after a lack of reading, is a weak structure. Without clearly defined, over-arching themes, your essay is doomed to be a collection of weakly related facts and observations. So it is important to know exactly how your essay is going to pan out when you start to write. As an exercise, try to outline your structure as a series of bullet points, each corresponding to a paragraph. Then group your data under the bullet points, so that the structure of your argument is ingrained in the text.
The Essay writing in Human Geography resource offers a whole section on 'organising your essay', which also applies to this context. The pages which follow here outline areas which are particular to writing in Physical Geography.