Okay, here is just a basic list of things I recommend doing/looking for when editing.
Read your story in a different way. On a Kindle or iPad, in PDF form, or even with a different font. It’s amazing what you can see when you’re looking at it in a different form.
Read it out loud, or have it read out loud to you. I’m now using NaturalReader to do this since reading it out loud doesn’t help me. This program reads the typos so they’re easily caught.
Be consistent. If you use “damnit” vs “damn it”, then use it consistently.
“Alot” is not a word. It’s “a lot”. Simple thing, but check for it, anyway.
Check for typos that might not be typos–like ‘they’re’ vs ‘their’ or ‘there’. ‘We’re’ vs ‘were’, etc.
Make sure you’re spelling your character names the same way, especially ones with unusual spelling. (This seems like a no-brainer, but it happens.)
Make sure your characters don’t have their eye/hair color change throughout the book. (I’ve seen this–especially with eye color!)
Check that your characters aren’t all saying the same exact same thing–like a special word or phrase. It happens in real life, sometimes, but it’s not going to happen with every character.
Do you use a lot of exclamation points? Are they ALL needed? If you use them for every little thing, they’re going to lose their impact, so make sure they’re really needed.
If you’re writing a series, especially in paranormal-like genres, keep your world building notes handy. If you’ve set up your world to do X, then it does Y, you better have a good explanation.
I have a few more things to add to this list, but these are the ones I could think of right off the top of my head. I’ll add things here gradually.
NaturalReader is the program I’m now using to read my novels out loud to me. Here are a few things you should know about it: There is a free version and a paid version. I’m using the free one for now, but there are some annoying aspects to it. For one, it pauses every few minutes (or about every 500-700 words) and pops up a buy-me message (that talks). Now, this wouldn’t bother me, except the pop-up message stays there, on top of your other windows, until you click “later”. But that’s about the only really bad thing I have to say for the free version.