You have worked on your article, poem, novel for a long time. You have put your heart and soul into it and now comes the hardest part of any writer's life...Proofreading. Most authors can write, but because the thoughts in there head come quicker than their fingers can type it makes it difficult to not make mistakes. When you go back and read it, it sounds great to you because you know what it is supposed to say and your brain skips over the mistakes.
This is why it is important to get an impartial person to read through your document. If might be a good idea to have the person sign a non-disclosure document before they start proofreading for you, this will be beneficial to both of you and if the personal is ethical then they normally won't have a problem signing such a document. Normally a proofreading will read through the entire document, if they notice mistakes they will flag them for later, but by reading through the document first they understand where you are trying to go and are better able to make sure it flows correctly. On the second go through the proofreader will normally read it through slowly to catch mistakes with flow and spelling. Then on the final pass they will go through the document word for word looking for spelling and punctuation. This time they won't be reading the document so much as scrutinizing it closely. By proofreading going through this process they are better able to check the mistakes that are made and to understand what you have left out or misspelled.
I know with my co-author, I will proofread the things she does and there will be times that we both are stumped on a word or phrase. We normally end up going back through it again to try and understand what has been said. We have found it beneficial to read through it ourselves first before we give it to a proofreader. If you can find even ten of the mistakes in the document then that is ten mistakes your proofreader doesn't have to stumble across. Now remember there will be things your proofreader will miss, they aren't perfect, but they will get the majority of them.
We have actually been very lucky in our proofreaders, several people have offered us their services free of charge. They were official proofreaders but they were editors for magazines and English school teachers. This has been very helpful to us and we greatly appreciate everything they have done for us.