There are several ways to say “said”, such as replied; asserted; stated; opined; declared, etc., but don’t overdo it; too much variety gets annoying as well. When expression is required, use substitutes to avoid adverbs, such as “he yelled” vs. “he said loudly.” The feeling behind it can also be described by how the character looks, his or her expression, or body language. Only a small portion of communicating is done via words.
Keep dialog realistic. Remember to use contractions or it could sound stilted. Saying it aloud helps. If one or more characters have an accent, be sure to reflect that as well, even if your spell-checker gags a bit. You want your readers to be able to virtually hear conversations. You should create an impression of what their voices sound like as clearly as you do their appearance.
In long conversations, remind the reader who’s speaking from time to time. It’s annoying when you have to go back and figure it out. You can do this in a more creative manner than “he said” or “she said.” For example, have one of them make a face, pace the floor, roll their eyes, or scoff.