How should I approach a history essay titled discuss the factors contributing to a particular historical event/phenomenon?

At A-Level, a comprehensive range of factors contributing to an event/phenomenon will be very impressive. In tryign to achieve this, however, it is important not to omitt any of the crucial factors- yes, everyone else will be putting these in their answer, but that is for a reason and it should be your first aim to provie a detailed accoutn fo these before broadening your answer to include other, less commonly noted, factors. 

In terms of structure, with your overall arguement made clear in the introduction (to be reiterated later in the conclusion), paragraph your essay into the various factors, although don't be constrained by this general structure if an answer requires more explanation. Each paragraph should consist of an anaylsis of the link between the factor stated and the historical event/phenomenon before the exposition of some selected evidence to convey the link. 

One way to make a strong answer excellent is to provide a heirarhcy of contributing factors. That is, either suggest a most important factor, stating why, or perhaps indicate a distinction between long run underlying factors and short term specific ones. Another approach which will raise the calibre of your essay is to play the devil's adovate in your own essay; this should be done in one selected factor and involves a nod in the direction of historigraphical debate by showing an awareness of an alternative point of view 'one might argue...' before explaining your chosen line of argument 'but it will be seen that...'. 

Good luck!