Good tips for self-editing:
1) There are MANY different acceptable styles, pick ONE and stick with it.
2) Finish your manuscript, let it rest a few days, edit. Let it rest again, edit. Let it rest again, edit. AND SO ON!
3) Beta readers can be invaluable, or not so much depending on who you select. Make sure you are choosing different people for different roles - content-oriented vs. grammar-oriented
4) Paperback proofs are your friends! Order a paperback proof, grab a pencil and mark away. It helps to read aloud, especially dialogue passages.
5) I personally find it helpful to edit as I write. After half a dozen chapters I take a small break to read back through and edit. This works for some people but not everyone.