What's a highly effective but relatively unknown proofreading technique?

Answer 1

Use your sense of feeling. Does the text feel right? If not, reread it in detail.

Answer 2

After editing, and proofreading, I suggest that you go to a quiet room to read the piece slowly aloud. I used this technique when I was an editor of an employee publication. This strategy was very effective to help find any additional and needed corrections.

Answer 3

Proofread the piece backward after proofing it the normal way. You'll likely catch things that you didn't notice as you were reading through it from top to bottom!