As a student, I wish someone would've helped me write more effectively BEFORE I turned it in. Instead, most of the learned I did about writing came as a result of pounding my head against a grader's pen and learning "that's not the way to do things!" As an instructor, I'm constantly seeking to help students avoid the same pitfalls I found. I found a list of several helpful suggestions from a law / theology student in California that I thought I'd rip off.
1. Never use a metaphor, simile, or other figure of speech which you are used to seeing in print.
2. Never use a long word where a short one will do.
3. If it is possible to cut a word out, always cut it out.
4. Never use the passive where you can use the active.
5. Never use a foreign phrase, a scientific word, or a jargon word if you can think of an everyday English equivalent.
6. Break any of these rules sooner than say anything outright barbarous
Or, in the words of the bumper sticker -- Eschew obfuscation, espouse elucidation! ;)
List taken from Pilgrim at Locustyears.blogspot.com