Think about your book and try to boil down the central character and plot elements into once sentence. Write this sentence down. This sentence should hook the audience.
Answer the following questions about your book: What is the genre? What is the tone? Is it lighthearted and commercial or thick and literary? Who is the main character? Who is going to read this book? Why are you the only person who could write this story? Jot these answers down as statements.
Using your notes, try and pitch your book out loud, using a conversational tone. Start with the hook and then launch into the other elements of the pitch. Practice until you have these elements memorized.
Ask a friend to sit with you and practice pitching in conversation. The best pitches are an interaction between an author and an agent or editor. Practice until you feel confident.
Research local writers conferences. Find one near you and register. Research the agents and editors who will be attending the conference and the works that they represent. Try and visualize your book as a part of their list and find examples of writers that they work with to compare your work to.
At the conference, find the agents and editors you have researched. Treat them like normal people and engage them in conversation. Try to get the agent or editor to ask you what you're working on by introducing yourself as a writer or novelist. Pitch as rehearsed.