I can spot a typo or misaligned text a mile away in someone else's document. After working on a project for a while, I'm no longer able to spot them quickly in my work. Here are some things I do to help improve my proofreading.
1. I proofread a paper version each time there are edits or major revisions. I miss things when I proofread on the computer screen.
2. If I'm tired, I put it down and come back later. I miss things when I'm not 100% focused.
3. Turn off the TV. I can't proofread if the TV or a movie is on at the same time. I do listen to music. I proofread best to jazz or light classical.
4. I often read aloud to check for errors or cadence problems in the dialog.
5. When I proofread, I read word for word. When you read fast, you tend to skip words. A pro told me once that to be very effective, you need to start at the bottom of the page and read each word (right to left). It takes the words out of context.
6. After my manuscript is edited and corrected, I ask several beta readers/proofreaders to go through my manuscript. The more eyes, the better.
7. When I'm checking the final copy, I look at everything (title page, chapter numbering, headers, footers, acknowledgments, etc.). I have had typesetters make mistakes or omit a header on a page.
8. These are my favorite editing/proofreading/grammar resources on Twitter. Check them out. They have the best advice on all things grammar and editing.