Since I am in the process of proofing Immortalis, I thought I might pass along a few quick tips.
1) Print it! Always proof a printed copy of your work. I can't explain why, but seeing something in print somehow makes you look more closely at it. I can spend hours staring at a computer screen, reading my work, and still miss things. As soon as I see it on paper, I find more errors.
2) Read out loud! When we read silently, especially our own work, our brain plays tricks on us. We know what we "meant" to say and our brain substitutes the correct sentence or words in for the ones we might have goofed on. Reading out loud, stops your brain from this sort of "auto correct" function. You will find lots more errors this way. If your computer has some kind of speech function, you could also try having the computer read to you. Sounds kind of silly, but will definitely help you pick out any problems.
3) Have someone else read it! Hey, sometimes we are just blind to our own mistakes. A fresh set of eyes works wonders. When you have proofed and re-proofed, toss your work to a friend and let them read. I bet they still find things for you to fix.