All excellent advice here.

I agree with the comment about 'get someone else to do it'. Ideally, get up to about four other people to do it, depending on its length and importance. Ideally your proofreaders should have the necessary skills and be aware of who your writing is intended for. They should also know what they are being asked to look for: spelling, grammar, style, layout.

Another trick is to read line by line, covering the rest of the text with something opaque. Spell check and grammar check are useful as long as you understand what they do (and don't) do.