I had an excellent A�level teacher who taught me what he called �the 5 point plan� for essays of 1000 words or less, and �the 7 point plan� for longer essays.
The premise of this was that all good essays should have an introduction (point 1) which should introduce the topic and explain (in brief) what your essay will look at, and a conclusion (point 5 or 7) which sums up your findings or arguments. Points 2-4 or 2-6 are then several good paragraphs in the middle.
He also suggested that you should always write your introduction AFTER the conclusion � that way you know exactly what you have written, and will be able to say with clarity in the introduction exactly what your essay is going to achieve.
I used this essay structure throughout university, and was always commended on my essay structure and coherency.
Good luck!