Conventional wisdom is that, when writing an essay, you should decide what your thesis is, determine what points best prove that thesis, use this information to prepare an outline, and then flesh it out into a full essay.
This was never particularly good for me, because either I had no idea what my thesis should be, or I had a brilliant idea for a thesis but couldn't quite pull the essay together.
So in university, I came up with another technique.
I started by opening a blank Word document and typing out everything I knew that was remotely relevant. Some if it would be in nice sentences and paragraphs, some of it would be in point form, some of it would be a list of questions to answer. I'd usually also have stray analogies and turns of phrase that I wouldn't mind working in there somewhere. I'd just braindump until my brain emptied, then put it aside.
The next day, I'd open it up again, read it over, add anything that occurred to me, and then figure out what thesis was most naturally proven by all this stuff I'd written.
Then I'd drag all the stuff on the screen around until it landed in the order that best proved the thesis, marking any gaps with "[...]" or "[talk about widgets here]" or whatever. Then I had my outline. And over half my essay.
If I had time, I'd put it aside overnight again, and then fill in those blanks I'd left the next day. After letting it sit overnight, filling in those blanks always seemed like a remarkably easy task. Just a few sentences here and there, no biggie! (If I didn't have time to let it sit overnight before I did this, I'd do all I could by brute force.)
Then another overnight, a fresh morning edit, and we're done!
If I didn't have time for more than one overnight, I'd do the braindump and determine the thesis on the same day, but with a break in between and in two different locations. (For example, braindump at home, spend an hour gaming, get dinner, then determine my thesis in the library.)
The result was an essay that does its job as well as possible. Because my thesis was supported by the points I knew most about, it was (very nearly by definition) the best-proven thesis I could come up with, and proven to the best of my ability. Essays written this way always got As, many of which were high As (at the university level), whereas essays written by choosing my thesis first more often got Bs, occasionally low As.