Academic writing, unlike creative writing, generally follows a set formula, allowing for a clear structure to an essay to act as the backdrop to your argument(s). Often, your course guidelines or essay title will define this structure but it will generally involve an introduction, some background to the main arguments, perhaps through a literature review, some analysis and then a conclusion. Although all of these elements are important in their own right, many would consider that a good conclusion is the mark of a good essay. The reason for this is that all of the earlier elements such as the introduction, hypothesis and analysis, are all reiterated within the conclusion, making the conclusion the summary of all of your research, theorising and review.
In order to create a good conclusion therefore the first rule is to ensure that all of the contributing elements are sound. Structuring your conclusion around weak evidence or research will mean that you will not be able to conclude anything with any real conviction. Secondly, a good conclusion must be relatively succinct. It is not necessary to totally re-write earlier sections of your essay or dissertation, you should merely draw out the key facts, pulling them all together into a sensible order. And finally, your conclusion should definitely summarise something, even if that is just to say that from the work you have carried out to date it is impossible to conclude in favour of one theory or another. This type of uncertain conclusion should not be considered a weak conclusion, providing that all of the contributing evidence suggests that your conclusion is in fact the right one.
It is very often the case that with lower level academia, or early research pieces, academics will identify gaps in their research that would need to be addressed in further study thus leading to an 'inconclusive conclusion'. In the same way the author may identify flaws in the practical execution of data analysis, perhaps realising biased interview questions, or closed interview questions that do not allow for the appropriate responses to be obtained. If this is the case then results may be skewed or the writer may be unable to draw any sensible conclusions. As long as you are able to convey all of this information then your conclusion can still be classed as a good one. Clearly, however, the ideal situation is that all of your work leading up to the conclusion is robust enough to allow you to draw an evidence-based, definite conclusion that leans one way or another.
Once you have written your conclusion then you should check your entire essay for spelling and grammar mistakes, and that you have followed the required style and referencing guidelines throughout. And most importantly double-check that your conclusion really does conclude something!