A few common issues I see with student writing are repetition, overuse of certain parts of speech, cliches and exposition and underuse of interior thought.
Here are some quick tips to help your writing flower:
1. Avoid repetition. Check carefully for repeated words in sentences and paragraphs and for stand-out words that repeat too often through a manuscript. Vary your syntax using semi colons (be sure to use correctly), colons, dashes and fragments. Avoid repeating dialogue tags like said (or replacements for it which can be even more problematic) by replacing with a description of what the character is doing as they speak.
2. Avoid overuse of adjectives, adverbs, prepositions and common verbs. I love adjectives but my first writing teacher told me to rely more on nouns because they will actually help the reader see things more vividly by bringing in their own experience. Adverbs tend to "tell" not "show" though a few original ones can be great. Try to avoid more than one in a sentence, or even paragraph. Avoid too many prepositions in one sentence. Try to pick unique verbs rather than relying too heavily on the ones we see most often.
3. When creating metaphors and similies, avoid cliches. One way to come up with fresh figurative language is to reference what has just happened in the scene. Does it give you clues to creating imagery based on what you have just described? For example, if you've been talking about cars on a freeway glinting in the sun, can you then describe someone's eyes by their metallic color?
4. Don't describe characters or setting in big chunks. Try to weave the description into the action and in between the dialogue as the characters interact.
5. Don't forget the interior thought. Even though you may know what a character is thinking or feeling, your reader might not. We can understand almost any behavior if we know the interior motive. Use descriptions of bodily sensations to show the feelings. Don't be afraid of a little well written exposition to tell the thoughts.