Take a break
Remember to take a break when you've finished writing your paper. After you have processed so much information, it can be highly advantageous to mentally distance yourself from your paper. Even as little as fifteen minutes will help clear your head and get you psyched to re-work your paper.
Read out loud
The next step in the paper-writing and revising process is to read the paper aloud. Yes, you may feel self-conscious doing this, but this step is absolutely essential to the writing process. Often, the eyes will skim over errors (particularly if you have been working on the paper for some time) If you find yourself stumbling or not understanding what you've written, there may be a problem. Reading out loud is a great way to catch comma errors, run-ons, fragments, and awkward sentences.
Take the time to slow down and examine your paper sentence by sentence to check that each is correct in structure, tense, grammar, and spelling. Each sentence's tense should be consistent throughout, and each subject should agree with its verb. You may want to try reading your paper sentence by sentence backwards. This will help you forget the larger picture for a while and focus on sentence-level revision.
Build on past proofreading
Try to recognize your common mistakes and write them down. For example, if you know you have problems with their vs. there, write down the difference on a piece of paper or a Post-It and keep it next to your desk when you're writing. You'll be saving yourself time in the end.