The said rule is two-folded. On one hand, a conversation that looks like...
"I like peanut butter." said Jim.
"That is good. I like jelly to be in my mouth." said Lisa.
"I like a lot of things in my mouth too." said Jim
"We should be friends for a while. And maybe we can eat peanut butter." said Lisa
"And Jelly." said Jim
...gets really tedious really fast. On the other hand, if it looks like...
"I like peanut butter." enunciated Jim.
"That is good. I like jelly to be in my mouth." Lisa warbled.
"I like a lot of things in my mouth too." sighed Jim.
"We should be friends for a while. And maybe we can eat peanut butter." roared Lisa.
"And Jelly." added Jim.
...it's still tedious. It is good to practice not overusing words, said included, but said does have its place. If somebody says something in a normal way, or in a way that isn't worth drawing attention to, and you still need to specify who is speaking, then said has its place. What you want to do is mix up sentence structure so that you don't fall into a monotonous pattern. Also, it is a good idea to drop the speaker indicator when you can. And, of course, it's always good to be descriptive, and a good descriptive dialogue indicators can set the tone for the rest of the conversation so that you no longer need them.
So, for me, a good dialogue looks like
"I like peanut butter." said Jim.
Lisa's eyes lit up and, like a child, she pointed wildly at her open jaw. "That is good. I like jelly to be in my mouth."
"I like a lot of things in my mouth too."
"We should be friends for a while. And maybe we can eat peanut butter."
"And Jelly." Jim added, giving Lisa a hearty thumbs up.