Something I aim to avoid in my writing (at all costs) is usage of the passive voice. It can sneak into writing and can create unnecessary distance between the reader and the MS. Not that it's wrong to use a passive versus an active voice (there are certainly situations where the passive voice is preferred - like scientific writing), but in general, text written in the passive voice can sound awkward and be more difficult to read than text written in the active voice (Storymill Publishing).
Usually, the passive voice is easy to spot - just do a search for every instance of "was" in your manuscript. Of course, "was" doesn't always indicate the passive voice. Sometimes "was" is just the simple past. Then again, any time I write something along the lines of, "She was walking", I like to take a second look to see if I can't change it to "She walked". So the "was" search helps me to revise on two levels.
Here are some helpful links with detailed explanations and examples of the passive voice:
The Writing Center
The Writer's Handbook
Do you ever find that you've written a sentence or two in the passive voice by mistake? Do you have any tips or tricks for catching these instances during revisions?