It's surprising how difficult it can be to find your own mistakes in your writing. I sometimes find that even though I am sure I have corrected everything I later see something I had missed. I'm working with some students who are finding it pretty difficult to correct their own work. Different things work for different people. The University of Reading has some useful tips on proof reading which I am going to be telling my students about, for example:
Print out your work - good idea, I definitely find more of my mistakes on paper than on a screen
Leave it a day - you are more likely to find your mistakes when you look at your work with a fresh eye
Take a look at the page from University of Reading here and improve your proof reading skills!