What worked best for me is this:
* before you even start reading, sit down and write a structure of your essay. This will force you to think about the topic, think about possible ways to argue and what your essay is about.
* after that do some reading - just read introduction and conclusions of papers - and get a general idea of the area. make sure to make some notes in your structure and amend as you discover new points / ways to argue.
* be open minded about revising your argument in face of new evidence
* write a general frame of your essay - i.e. now that you have done some reading and made notes, expand on each point a little and think about a coherent structure to put everything in
* finally, once you have a structure and your argument sorted, work on each part of the argument in more depth - i.e. do further reading, read papers and try to expand on each section
* once you have expanded on each section and written all parts of your essay, go over everything again and streamline it, strengthen the arguments, remove unnecessary words, etc.
* when you are completely finished: make sure to rewrite the introduction and conclusion - this is the first thing your markers will read and they should be perfect. if you changed arguments, etc. now is the time to rewrite your introduction!
I generally found structure and coherent flow of the argument to be very important in getting high marks. You can have the best argument but if it's super hard to read and doesn't flow well you will not get high marks. (obviously make sure your argument is actually good, if your argument is terrible even the best structure won't get you high marks - although it will probably improve your marks somewhat).
Source: three years of undergrad essay writing with mostly fist class marks.
If anyone would like some examples of good essays, send me your email in a PM and I can send you some of my essays as examples.