I got 80% on one essay, but it wasn't anything really great, I think it was just a nicely worded question and I liked the topic. I did like the essay though. I think sometimes it's hard to get very high marks in essays partly because of the word counts - the balance between including enough depth and having to cut down some information and choose the most relevant information to fit the word count.
I got 96% on one Stats assignment and 80% on an SPSS exam but again, they're easier because they're more 'closed' rather than being more subjective like essays are.
In my second year essay exams I got 2x 80% and one 90%, which I was very very happy with, especially considering that we'd only just found out my mum had cancer then, and in the one week I went back to uni for exams the police took me to hospital 3 times because I coped so badly. I think I did ok in those exams just because of the amount of revision I did though, I used revision as a way of avoiding what was going on at home! And being lucky with the questions maybe, the core textbooks were very helpful.
In essay exams I think there's a few things that can help, mainly just remembering the structure etc - a good intro/conclusion and 3-4 main points that you can expand on and evaluate well, just like you would in any other essay. And things like there's no point in trying to remember every reference and researcher, so just consciously revising the key names and then key pieces of research that you can include, or the key details rather than exact statistics and data. And quality over quantity too - if you write a great first half of an essay but then ramble on trying to include everything even if it's not completely relevant, it's going to balance out as a lower average compared to an essay which is shorter but includes just the good relevant information.