As an editor, establishing a good editing relationship with the author is of primary concern. Without trust between you, editing is going to be a disaster. The author needs to know you're interested only in making their work shine, not changing their voice, style or story. When an author knows this, they will be much more likely to accept suggestions and changes. So work hard at making your author(s) understand this and give them plenty of reason to trust you with their "babies."
1. Grammar, spelling and punctuation are paramount. If you're not sure, check it out. So if you have problems with lay and lie, its and it's, affect and effect, make sure you look it up. There are plenty of online dictionaries and places to check punctuation and spelling. Bookmark your favorites and keep them handy while you're editing. And when you change something, remember to educate your author as to why you're making the change. If you make an author aware of the difference between you're and your, they will be less likely to make the mistake again, and you (or another fellow editor) will have less to correct in their work in the future.
2. Watch for awkward phrases and unclear ideas. If it doesn't make sense to you, it's not going to make sense to the reader. Have the author clarify or rephrase.
3. "God" does not have to be capitalized if it does not refer to the actual deity. So "Oh my god that feels good!" would be acceptable. However, it would be, "I prayed to God that he would get better."
4. Purge the words "just" and "that" as much as possible. They are often unneeded. For example, in the previous sentence, the word "that" is unnecessary. It should read, "I prayed to God he would get better." However, the "that" in the former sentence, "Oh my god that feels good!" is necessary. Smoothness/readability is the most important guide here.
5. Here's a short list of words you should probably purge as much as possible from most works: a bit, about, a little, actually, almost, already, appears, approximately, basically, close to, even, exactly, fairly, finally, just, kind of, mostly, nearly, now, pretty, quite, rather, really, seemed, seems, simply, slightly, somehow, somewhat, sort of, suddenly, truly, utterly, very
6. Minimize the use of "And" and "But" to start sentences, unless it has a dramatic use in the scene.
7. A scene break should not occur if the action is still contained within the same time frame and location -- unless it's being used to indicate a point-of-view switch.
8. Highlight repeated words and have your authors choose alternates if you find them often repeating the same word throughout the mss. Also, avoid repeating words within sentences, paragraphs, or even succeeding paragraphs. Please be consistent in your highlighting, however, and explain your system clearly to your authors. (For example, one of our editors uses green highlights for repetitions and redundancies, yellow highlights for important changes, blue highlights for drawing attention to something that should be changed, etc.)
9. Correct passive voice, especially past progressive voice, as much as possible. "She was sleeping peacefully" should be "She slept peacefully". Active voice gives a reader a more immediate sense of what's happening. This isn't a hard and fast rule, but a general one, and I admit, a trendy one in the current market. Passive voice can be effective. Use your best judgment.
10. Measurements and numbers should be spelled out—pounds for lbs, ounces for oz, two for 2, etc. The exceptions to this are years: 1970 is better than nineteen seventy, and time indicators should be a.m. and p.m. (not capitalized) with a space between the time and the indicator: 2:00 p.m. This should, however, be a last resort. Try to rephrase to make it "two in the afternoon" or something like that.
11. Stephen King said the road to hell is paved with adverbs. Eliminate those --ly words as much as possible: beautifully, sexily, whinily. Ugh. Verbs and actions are always better to convey meaning. Adverbs are an emphasis technique and can easily be abused. A few adverbs here and there are fine, but if your author is an adverbaholic, it's time for a little AA -- Adverbs Anonymous. Highlight all the adverbs in the mss to give your author an indication of how many they're using and ask them to rephrase.