How do I write a good conclusion to my essay?

Writing a good conclusion is one of the hardest parts of an essay. Remember that the conclusion is the last impression you give an examiner, so it's very worth putting in extra effort and thought, even under timed conditions. The temptation is always to go over and summarize all of the points you've just made in the body of your essay: resist this temptation - it's dull to write and it's even more dull to read. Try to make your conclusion a punchy round-off to the argument of your essay, and be creative! Good ways to do this are to re-analyse a particular quotation as a "send-off" for your general thesis, or to turn your argument "outwards" by examining it in a wider context (though be careful not to do any "arguing" - the conclusion isn't the place for this, and by this point your argument should have already been made). For example: if you've been writing an essay about how Chaucer uses dream visions in The Canterbury Tales, you could frame this in the conclusion with a brief statement about what Chaucer does differently to dream visions in other medieval texts - whereas dreams in Julian of Norwich or the Pearl-manuscript are revelatory and impart general "truths", Chaucer toys with this convention by making the "truths" imparted provisional or unclear, often with comedic effect. There is an art to this, and writing a good conclusion will take practice! Just try to think about how you can send off your essay without going over the whole thing again, and without over-simplifying your argument. Stick to two or three sentences, but make them count! 
     Your conclusion should have a sense of having reached a destination, and this is only possible if your essay sticks together well as a whole: the most well-written conclusion still can't pull together an essay which doesn't have a clear direction to start with. As ever, planning is absolutely vital - every part of your essay should have something to offer in demonstrating your thesis (which should itself be stated in the introduction). If you've planned well and your essay has a clear progression in argument, then your conclusion will be much easier to come up with. Think of it as the icing on the cake: the substance should be there already, the conclusion just rounds it off and makes it look tasty!