Give yourself some time
If possible, let your work sit for a while before you proofread it. If you are able to clear your mind and approach the writing from a fresh perspective, then your brain is more able to focus on the actual words, rather than seeing the words you think you wrote.
Proofread a printed version of your work
If I proofread on a computer monitor I always miss more errors than if I print out a copy and go over it on paper.
Read your work out loud
This forces you to read each word individually and increases the odds that you'll find a typo. Reading it to a friend, or having your friend read it to you, is even better.
Use a sheet of colored paper or a ruler as a guide while you edit. This sheet will help you keep your eyes fixed on the lines; there's something about color that seems to sweeten the senses. Use a colored pen when making corrections too.
Watch for contractions and apostrophes
Very often errors are made with words like you're and your, their and they're, and the most common - its and it's. Use the Find option in your Word program (CTRL+ f or cmd+ f on Mac OS) and search for ‘. You will find all the places that use the apostrophe and thus, all the contractions.
Enlarge the text or change the font
Changing the font may make it easier to spot mistakes.