1. By withholding information from your reader you do not create mystery and suspense. i.e. by saying that your character is hiding a heavy object in her skirt and then revealing it is a gun later–you probably are just annoying your reader. If you had simply said she had a gun in her skirt and was following a man that would create tension and pique curiosity. Vague is not interesting. Concrete details are. (There are always exceptions to these rules, I know.)
2. Adverbs stand out and look amateurish. Think long and hard about using an adverb, and use them with a light hand–if you must. Sparingly indeed.
3. Dialog is the most valuable real estate in fiction. Use it wisely. Do not put anything in dialog that can be said in the narrative. Attribute dialog, because long pages of unattributed dialog are annoying and hard to read. When you attribute dialog refrain from things like: he yelled, he laughed maniacally, he said with a grimace, he yodeled, etc. Those sorts of attributes come off as cliche, and also remind the reader that they are reading. Either a simple “he said.” or “name said” works, but you can also add a beat to show what is going on, where the character is, or what he is doing and leave out the “said” all together.
“I hate you.” He started to peel paint flakes from the window sill and eat them.
But you have to be careful with that sort of beat in a dialog. It would be easy to make your characters hyperactive. Beats should be used sparingly to break up dialog and really to illuminate it subtly.
4. Vary the rhythm and structure of your sentences, especially when you want to stress something. This has been a problem of mine. I am not a poet, but I seem to be able to write metered phrases easily. Prose with a consistent meter gets monotonous to read. And you want your story to have ENERGY.