Have you ever noticed how many mistakes you miss in your proofreading? It's frustrating to get a graded paper back and realize that you lost points because you left out a word or because you used too much passive voice. These are things you should see when proofreading, but it can be difficult to proofread ourselves. Our brain sometimes "sees" what we mean to say instead of seeing what is actually on paper.
It's a good idea to use a text-to-speech program that will enable you to listen to your own writing. After finishing a draft of your paper, read it over to make sure you don't have word mix-ups. Then listen to your essay to find out if you've left out any words.
You may have text-to-speech on your computer now. If not, you can find a free download online.