Proofreading should be reserved for when there is something to proof, which means you should be finished with your writing before you hit that backspace button even one time. If you have Microsoft Word, make sure your grammar check is not turned on, because nothing kills the flow of writing like the appearance of those squiggly lines. They are enough to drive any sane writer crazy, no matter how good your grammar and typing may be.
If you stop to proofread after every sentence, or even every paragraph, your momentum is disrupted and you have to regain your thoughts every time you start again. By waiting to proofread until you are through, you will keep a steady flow of consciousness and your words will reflect the continuity. If you are writing a single article or post, wait until you are completely finished to proofread. If you are writing something large, like a book, wait until you have finished a page or chapter before proofing.
So, in order to maximize your efficiency and keep continuity in your writing, hold off on proofreading or making corrections until you have completed what you are working on. Not only will you write faster, but your edits will also be more accurate, improving the overall quality of your writing.