Read a few hundred pitches or a few thousand for common pitch language shortcomings. Walk seven hundred miles in an agent's shoes, in other words. authonomy.com is a one-stop shop for tens of thousands of droll pitches, one or two rough gems among the fray.
Recognize a query is a letter of introduction for a literary entity. That the introduction is about the literary property, not about the writer.
Use active language.
Use third person and present tense voice.
Clearly, concisely, briefly express what the central situation and dramatic complication are, what the central want or problem wanting satisfaction is, and what opposes satisfaction. But leave room for interpretation.
Demonstrate that full due diligence has been done. That the agency represents the genre of the property at least. That the agency welcomes properties similar to the property. Whether the agency accepts reworked properties or not. Etc.
Use the attitude of the main attitude holder of the property. If she's ironic, write the pitch ironically. If he's sarcastic, write sarcastic. If it disapproves of the novel's topic, write disapprovingly. If he, she, or it is unreliable, tentative, or uncertain, write so. And so on.
Entice, excite, engage empathy and curiosity; tension, in other words, but don't telegraph the ending or give away the plot.
Use everyday conversational language.
Injudisciously use prepositions, conjunctions, fuzzy parts of speech, syntax expletives, passive voice, dangling modifiers, lengthy sentences, invariant syntax, or negation statements.
Write a pitch the same way, in the same dry voice, in the same unemotional attitude as millions of other query writers do.
Claim the property is literary fiction. Others make that distinction later.
Use rhetorical questions. A rhetorical question artlessly tells readers what a central dramatic question is. Do express the question artfully but leave room for clear yet interpretable intent and meaning.