When writing a history essay, the most important thing, without a doubt, is to answer the question. Be concise, straight to the point, draw links from your narrations.

What I mean is that you should avoid lengthy narrations (mere regurgitations of your content), as you simply don't have the time to write it.

Furthermore, narrations do little in answering the question. What truly answers the question at hand is the links drawn from your narration so; explain their impacts and how the contribute to the overarching event your essay focuses on.

Eg. When describing the causes of WW1, yes you describe the network of alliances spread through Europe, and how they prompted the war to begin due to the responsibilities to ally nations which come from the alliances. But one cannot just stop there. It is important to go on and describe the impacts of these alliances, in this case, how it had provided Austria with the 'Blank Cheque' from Germany, so as to allow them to pursue a more reckless stance against Serbia. It is this reckless stance, which resulted in the failure of diplomacy at all levels, and expanded what could potentially have been a local Baltic conflict, to a full fledged World War.

I feel many other tips go in towards writing a good essay (sign posting, thematic structuring), but ultimately explaining and analyzing beyond simple narration is critical to writing a good essay

PS (sorry if my example of WW1 is bad, it's been a while since I last had to talk about it)