One of the books I'm enjoying at the moment uses a lot of synonyms for "said." They also add extra descriptors to the verb, sometimes making the description of HOW they said what they said (expression, tone, breathing style, etc.) longer than the spoken sentence.
Example: "No," he said quietly, jaw out-thrust, eyes half-lidded, crossing his arms.
Let me give some advice that I've received many times: cut out everything after "he said." Throughout the character interactions, body-language can be peppered in. If the "No" is an immediate response to someone else, then you could write it like: "No!" He crossed his arms and glared.
Now I could give you dozens of examples and recommendations on how (I think) dialogue can be improved, but there are dozens of books and probably thousands of blogs dedicated to the subject. My advice will remain simple.
Just use the verb "said." Synonyms can get distracting, but "said" will eventually get ignored by the reader because they're moving forward with the story. And if every sentence of dialogue ends with "he said," "she said," or "John said," you don't have enough body language or action taking place within the scene.