Speech tags
Use said and asked. If the situation calls for it, you can use yelled or whipered.
Do not use questioned, riposted, retorted, queried, uttered or other synonyms. It's annoying, distracting and the mark of an amateur.
If your character hisses anything, make sure there is an S in it. You can't hiss, "Never!"
Dogs bark. If your character must bark, make sure it's something short. Same with growling.
Adjectives and adverbs
They exist in English for a reason. They are not the work of the devil. But use them where they will do most good, don't sprinkle them throughout your narrative.
Don't use adverbs to replace good dialogue.
English is carried on the verbs. If you find the strongest verb, you don't need a string of adverbs as well.
Limit adjectives to one per noun. Since adjectives go before the noun, your reader has to memorise all the adjectives on the way to the noun. "The tall, strong, blue-eyed, agile schoolgirl" requires a mental jump as the reader adds the adjectives to the noun.
General
Do not use "started to" or "began to" unless the action was not completed.
Keep action direct. "She heard the sound of a car outside" is stronger as "A car pulled up outside".
Examine any sentence that includes felt, heard, seemed to see if you can cut them.
You can almost always cut: suddenly, just, absolutely, very, completely, totally, a bit, about, a little, actually, somewhat, a little, that, quite, really, truly, very.
If you ever find yourself using the word "somehow", it usually means you have a big hole in your plot.
Redundancies
The bald-headed man. We assume it's his head which is bald.
Her heart beat in her chest. Where else would it beat?
He shrugged his shoulders. What else could he shrug?
She thought to herself. Unless she's a telepath, who else could she think to?
Repetition
The easiest way to avoid repetition is to read your story out loud. You'll hear if you are overusing a word or phrase and can fix it.
