Things I've learned from trying to edit my story:
• If you've read a passage 10 times through and still can't see where it needs changing but know it needs to be changed, highlight it and come back to it later with fresh eyes.
• A change of font can help you see things you hadn't picked up before.
• If you ask your family to proof read for you, don't expect them to pick up everything that should be changed.
• Make sure anyone you ask to proof read has a good grasp of grammar and spelling, and make sure it's the same spelling you use! If you're British and you ask an American to proof your story, don't expect them to pick up your spelling mistakes.
• Things always look different on a printed page than on a computer screen.
• Something that makes perfect sense to you may not to a reader who hasn't spent the past two years engrossed in the story...
• If you ask for honest feedback, don't be offended when you get it! (Unless it's completely useless, like 'your story's really bad, I didn't like it' with no suggestions to improve it.)
• Treasure the good comments but don't be afraid to change things.
• Always keep a copy of your previous draft, just in case!