How can I clarify my thesis statement on New Historicism's effect on "The Necklace"?

Indeed, New Historicism evaluates how "The Necklace" is influenced by the time in which Guy de Maupassant wrote it through the struggling of the lower class and poor economic condition.

The thesis statement is the most important sentence in your essay.  It should tell your reader what you are writing about clearly and concisely.  A clear thesis statement is direct and to the point.

You have three main points you are trying to make in your thesis.  First, it should be clear what New Historicism is.  Second, it should be clear how New Historicism relates to the story.  Finally, you need to relate specific events to the theme of the story as seen from New Historicism.

New Historicism is a literary theory focusing on the context in which the story was written.  In other words, you cannot take the story out of context.  The story has to be understood as influenced by its time period.  Therefore, you need to say so in your thesis statement.

From New Historicism’s standpoint, “The Necklace” is a product of a society obsessed with wealth and status…

This is just an example.  I chose this example based on what you included in your first thesis.

Now that we have been more specific on New Historicism, we need to address how this relates to the story.  You are halfway there when you mention “the struggling of the lower class and poor economic condition” (that should be conditions).  You need to be more specific though.  How does the struggle of the lower classes relate to the historical context of the story?

From New Historicism’s standpoint, “The Necklace” is a product of a society obsessed with wealth and status, where the struggle of the lower classes to overcome economic hardships …

We are almost there.  I clarified the “poor economic condition” and “struggle of the lower class” a little.

Finally, you need to relate this to some kind of theme.  I would like to draw your attention to a quote from the story.

She dressed plainly because she could not dress well, but she was unhappy as if she had really fallen from a higher station; since with women there is neither caste nor rank, for beauty, grace and charm take the place of family and birth. (enotes etext pdf p. 1).

Think about how powerful this statement is.  She is not happy because of her station in life.  She was not born rich, so she cannot dress well.  She equates dressing well with social status, because society does.  She is therefore a product of her society.

From New Historicism’s standpoint, “The Necklace” is a product of a society obsessed with wealth and status, where the struggle of the lower classes to overcome economic hardships is represented by Mathilde’s obsession with material wealth.

You would go on to explain that because of this obsession with material wealth, Matildhe does not notice that she has wealth in other ways, in terms of a supportive husband, health, and good looks.  She loses all of these things by the end in her quest for material wealth.

How can I improve my conclusion for an essay about New Historicism's influence in "The Necklace"?

In conclusion, New Historicism assesses the construction of Guy de Maupassant through the short story, "The Necklace". It also stresses its role in the historical and social context. The author provides us with the struggle of the lower class and poor economic condition.

The most important thing to remember about your conclusion is that it should mirror your introduction by focusing on your thesis.  A conclusion can also include a little extra flair, including a summary of your basic argument.

It is a good idea to first return to your thesis when writing your conclusion.  Here is your sample thesis statement.

From New Historicism’s standpoint, “The Necklace” is a product of a society obsessed with wealth and status, where the struggle of the lower classes to overcome economic hardships is represented by Mathilde’s obsession with material wealth.

The main points in this thesis are your argument.  Basically, you are saying that New Historicism suggests that the story “The Necklace” is influenced by the materialistic culture of France at the time. In your conclusion, you should focus on how you developed these points.  You need examples from the text of the influence of society on the characters, especially the social-climbing Mathilde.

In the body of your essay, build your argument and then support it with evidence.  Here is some evidence of Mathilde’s ambition.

Mathilde suffered ceaselessly, feeling herself born to enjoy all delicacies and all luxuries. She was distressed at the poverty of her dwelling, at the bareness of the walls, at the shabby chairs, the ugliness of the curtains. (enotes etext p. 1)

It seems like Mathilde is comparing herself to people with more money.  You need to stress how this relates to your thesis in a body paragraph of your essay.  In your conclusion, after restating your essay (and possibly changing the wording around), you would summarize your points.  You can refer back to this argument as follows:

Mathilde is not pleased with her status.  She compares herself to the better off Madame Forestier, whom she sees as her superior only because she has more money and therefore better possessions.

Of course the irony is that the necklace is fake.  You would explore that in a further paragraph, and add that to your conclusion.

A well-written conclusion is concise, like an introduction, but also reminds your reader where you have been in your essay.  Restate your thesis and review your points and you’ll be fine!