Keep your sense of time consistent. If the story takes place in the past, make sure your adjectives and modifiers are matching that past tense. Also keep in mind proper and basic grammatical rules for spelling and punctuation. Be sure to use as many words in your vocabulary too, and not reuse the same word a few times to describe an action that could be wildly different from every other action up to that point. Thesaurus are good for expanding vocabularies, and any old highschool text can help you with proper English grammatical rules.