First, let's distinguish between editing and proofreading--they are not synonymous! Editing is the process of reviewing for content, logic, and other high-level elements, while proofreading is reviewing for misspelled and misused words, mechanical errors, and punctuation mistakes. Editing and proofreading are, in fact, two distinct processes that should not be conducted simultaneously.
As we all know, proofreading is important. No one wants to submit a document with misspelled or misused words, as we all understand that this can make us look foolish, less than professional, and downright lazy and/or incompetent. Unfortunately, however, many writers depend solely on their spelling or grammar check to find errors in their documents. While the grammar- and spell-checks are great tools, they shouldn't be the sole method writers use to find errors in their documents.
Let's discuss a few practical tools you can use to proofread. One of my favorites is to read my document out loud. In fact, when I do this, I'm always surprised at the errors I find, even after I feel that I've carefully proofread the document. While reading aloud can be problematic if you work in a cubicle, it is a great way to find mistakes.
Read the document backwards. Sounds weird, I know, but this is a great way to find errors. What you do is to simply read the document from the end to the beginning. This method is especially helpful if you've been very immersed in the document and are sick of looking at it!
Change the medium. I tend to proofread online, so to ensure that I catch everything, I also print a copy of the document and proof that as well. Changing the medium can be a great way to catch mistakes.
Change the font. If you do proofread (or edit) online, consider changing either the font itself or the size of the font. For instance, if the document is written in Times New Roman 12 pt, change it to Calibri 11 pt. You'll be amazed at the difference such a small change can make in how you see things.