I've been editing my own books and critiquing several other people's over the past few weeks. There's one saying that keeps coming back to me:
SHOW, DON'T TELL
What does this mean?
Well, I'll tell you.
No, wait, I'll show you.
Tell: Kate felt angry.
Show: Kate's gut clenched, and she suddenly wanted to hit him.
Showing makes everything more vivid. I've also seen a lot of conversations summarized rather than dialoged. Give us the conversation, unless the characters are telling other characters about something we, the readers, already know, e.g., "Dan brought his parents up-to-speed on the whole bathtub-in-the-tree situation."
Show us what the characters say.
Show us how the characters feel.
Show us everything. But only once.
My favorite show-don't-tell quote is from Anton Chekhov:
Don't tell me the moon is shining; show me the glint of light on broken glass.
It always gives me shivers to read that. I may have to embroider it on a pillow or something.