I am currently buried under an enormous pile of proofreading at my paying job, but I thought I'd pop over here with a recent insight from the experience.
Changing page layout will make a text read differently. I am always, always finding new errors when I proofread digest-sized pages of material that I'd already proofread on letter-sized pages. This is partly because the eye tracks differently on shorter lines. I've also noticed that some spatially-related issues like paragraphing (specifically, overly long blocks of text) are much more apparent on smaller pages.
I highly recommend that you do your final manuscript proofreading in a size other than letter size to give yourself fresh eyes. It's pretty simple to do this. Go to the "page layout" menu in Word, select "paper size" and choose "A5"--which is roughly digest size. When you're ready to print, open the print menu, go to the "pages per sheet" drop down menu in the bottom right, and select "2 pages."
You'll be surprised how different your manuscript looks--and how many errors slipped past you when you always saw pages in the same size draft after draft.
What other tricks do you have to give yourself a fresh perspective?