You can use a thesaurus to jog your memory when you're trying to come up with a better synonym, but never use a word with which you aren't already familiar. Words often have connotations and nuances of meaning that you can appreciate only after having seen them in context, so you're taking a great risk if you use a word that you don't know well.
Even if you do feel comfortable with more advanced vocabulary, you should use the simpler synonym if that captures your meaning just as well. For example, instead of "ameliorated the situation," you could just as easily say "improved the situation." On the other hand, a word like "exasperated" is more intense than a synonym like "frustrated," and so you should use it if that's the sense you're trying to convey.
Choose words that capture your experience fully and accurately. For example:
VAGUE: When we first started the business, I performed a range of duties to get the company going.
PRECISE: When we first started the business, I took the initiative to contact potential partners, evaluate the service of our competitors, and tailor our plan to local markets.
Inexperienced writers think that using fancy adverbs and adjectives will make their writing look more eloquent, but in fact they just bog down your rhythm and usually sound like fluff. They also tend to make your writing sound abstract because they are not actual physical substances. Good writers stick to concrete nouns that the reader can grasp, and even more importantly, vivid verbs that are the lifeblood of active, engaging language.
BEFORE: I ran quickly to the board where the results would be posted, with many curious people standing around waiting anxiously to see their scores.
AFTER: I rushed to the board to find people crowded around muttering prayers to themselves as they awaited the dean's arrival with their score results.
The phrase "ran quickly" has become the more succinct and punchy "rushed." Instead of "many curious people standing around," we have substituted "people crowded around muttering prayers to themselves." Thus we gain a more vivid verb in "crowded" and a concrete image of people muttering prayers instead of the abstract adjective "curious" and the clunky adverb "anxiously." In focusing on nouns and verbs, we have succeeded in showing instead of telling.
Don't use words twice in close proximity, and don't use the same words regularly throughout an essay. The problem usually comes in overusing the same noun that's central to your topic. Although we emphasized the importance of precision when you're describing the details of experiences, you can get away with synonyms when writing more broadly about themes and topics.
For example, if your essay is about your skills in interpersonal interaction, you could use such similar phrases as "communication strengths" and "building trusting relationships."
Next: Verb Tense