The whole point of proofreading is to see if it will look right and make sense to the reader. So by definition it means getting a new pair of eyes (i.e. another person) to do it! Period!

If you are proofing someone elses finished product versus their rough written draft, then a good 'trick' is to place the copy side by side and literally use your two index fingers running underneath the words one at a time, reading along and comparing the two.