Its amazing that however many times you have read and reread your work there is always at least one mistake, whether it is grammar, punctuation, spelling, inconsistency of facts or typo.
Apart from the fact that errors make it appear that you have not carefully read through your work or that you are not observant enough to spot offending blunders. It can actually change the meaning of your text, or confound what you were trying to communicate. You can try correcting errors as you work, but when you put the whole document together you will still have to make corrections.
Sometimes the more you read through a piece of work the less likely you are to spot any glaring errors, in your mind what you are reading is what you intended to say, (familiarity breeds errors). How many times have you written an email only to find that when you get a reply and you review what you actually wrote - you didn’t actually write what you thought you had.
When writing an essay, project or theses, always consciously build in time to proofread you work, not just the hour before you need to hand it in or press send, and never rely totally on the spellchecker, as it may have used words that you had not intended.
You need to systematically work though a proofread.
Don’t print out the document every time you proofread - as you will need to enter any changes to the document which may result in typos.
For the purpose of proofreading change the size of the font and also double or triple space in the print preview. By doing this you are not just looking at the word shape you are actually looking at individual letters.
Now look at each line of text separately. This will enable you to pick out errors that may have been passed over in a first edit.
Cross check and verify your references. This may not be as time consuming as you think as you will have kept all your notes in a project diary.
Are you missing any pieces of information ? Do you need to smooth over links between ideas and concepts? Are there any inconsistencies?
When you have made all the changes after following the above steps - you need to read through your work again. Making a change in one or two words may require a change in tense or ownership in subsequent sentences.
Make sure that you have give as much priority to middle and end of your document as you do to the beginning.
Some times it may be appropriate to pay a proofreader especially if you document is very long. But be aware that if you pay someone, its still your work, so you still have the responsibility if your project does not get the grade you had hoped for.