If your essay responds to a prompt, you are well advised to use the words and ideas in the prompt frequently throughout the essay. This shows that you have thought carefully about the prompt, that you are addressing it directly, and that you did not plagiarize. If there is no prompt, give your essay unity by continuing to use words that express the thesis.
Here are several words you should seldom use in academic essays, although they might be perfectly acceptable in certain contexts and in other kinds of essays:
(1) "totalizing" words such as always, never, everyone, all, every, everywhere, totally, absolutely, and so on. These words are hard to defend, because your readers tend to be good at finding exceptions.
(2) "conversational" words and phrases such as you know, I feel that, I'm trying to, what's up with, and so on. This guideline includes almost all slang and almost all contractions. Note, however, that an essay should sound "smooth" when it is read aloud.
(3) "judgmental" words such as stupid, dumb, awful, terrible, great, amazing, and so on, unless you explicitly defend your judgment. Some demeaning words such as dumb can almost always be specified better by using a less demeaning, more precise word.
(4) socially or culturally "unacceptable" words. In some settings, calling an adult female a "girl" is appropriate, but in other settings it is inappropriate. Context often makes all the difference. Writers of academic essays are often taught to write in "gender neutral" terms whenever there is no reason to write about males or females in particular, and more and more readers are requiring that this norm be followed.
(5) Recognize that your words can have ideological meanings that please or anger your readers: the rise of the term "Democrat Party" in place of "Democratic Party" might tempt you to write "Democrat Party" in an essay about politics, but this trend has been limited mainly to political conservatives, so your use of the term will go over very differently with different readers. Whenever there is a genuine question about what is appropriate, choose the less controversial term. An academic essay about something else is not the place to fight an unrelated social or political battle, tempting as it may be.
The section "Five Ways to Turn Off the Reader" under "What Makes a Good Essay?" gives further advice.