Conclusions have always been my strong point (not sure whether this is a good thing or not haha), basically what pinkii said. You don't need a prepared conclusion because the ones made on the spot are stronger because you'll probably have included more information than a pre-prepared essay's conclusion. First line you refer back to the question, then go through all of each paragraph briefly and discuss (or tie) all of it together to explain how it relates to the question, if that makes sense. Most of it can be tied together with a really strong concluding statement. This is particularly strong (like Galladrin said) in giving you a good final mark.
Finally, make sure you just conclude an essay if you're writing out of time. A concluded essay that's half finished is much better than one that just cuts out.