[Quoth]

Read loads of books. Also outside your genre.
Take a break to forget the book after writing/edit.
Proofread on an ereader. Certainly never on a laptop or tablet.
Proofread and edit loads of lesser known gutenberg texts. To edit, export as RTF from Calibre and then save in WP native format. Do a Save As in "docx" to re-import to Calibre.

WARNING: I'm rubbish at Pearl specifically, and Regex in particular. Older MS Word might not do these, but LO Writer is free, and you can interchange with Word if your styles are done properly. In last resort use RTF.

Learn Regex to find stuff not obvious in reading (examples for LO Writer):
Replace all tabs \t with spaces.
Replace all pairs of spaces with space till none found.
Check that — and … against quotes have the correct quote type. Bug on some Wordprocessors.

Do these next two in turn with Find Next only. To see if dialogue after a speech tag after dialogue is wrong. In LO Writer you need to tick Case Sensitive too.
first \, “[A-Z] (Note an Uppercase MIGHT be correct)
then \.” [a-z] (Note a . if the text after closing quote is an Action is correct, but if it can be construed as a tag, then it's likely lower case and . should be ,)


Find with NOT case sensitive. It should find headings only as regular text body paragraphs always end with punctuation.
[a-z]$

Other errors (Find with Case Sensitive), like missing spaces or erroneous spaces:
[a-z][ ][“] [“][ ] [ a-z][”] [.a-zA-Z][“] [,.][“a-zA-Z] [ ][\.] \.” [a-z] \, “[A-Z]

Probably someone expert at regex can think of more, or combine some or do it better.
Regex search and replace with these to remove leading spaces, trailing spaces and blank paragraphs
^[ ] [ ]$ ^$

The ideal is to have someone expert proof read.
A good proof reader and a good beta reader are not the same person.

If editing in annotations from proof reading, Save As an incremented version. Only edit in the typos. Mark passages to be deleted, rewritten or added with a ¬ or something. Save As and increment version
Always Save As and increment before deleting a passage.
Always make backups.
Always Save frequently and have Auto Save on, but at about 15 minutes.