8. Coincidence kills plausibility. Don’t let a one-in-a-million event rescue your protagonist from trouble, or readers will stop believing that this person is truly affecting the course of events.
But what if you NEED that to happen? Well, make it happen. don't just let coincidence take over. Use "cause-effect" to get from "I need" to "it happens."
EXAMPLE: in my book Poetic Justice, I needed John to meet Jessica early in the story so that they can start their alliance to save the Shakespeare manuscript from destruction. When I started writing that section, I had that they "just happened" to meet at a party and start talking about their mutual love of Shakespeare. But right away, I knew that was lame. So I backed up, and made this meeting be the effect of some cause-- in this case, John learns that Jessica might have access to the lost Shakespeare manuscript, and sets out to meet her. She might think she just happened to meet the one other person in England who knows about this manuscript... but in fact, he planned the whole encounter.
One real benefit of making a coincidence into "causation" is that your scenes will be a lot more interesting. I went from one scene of John just stumbling into her at a party, to three scenes, the first where he finds out about her connection to the manuscript, the second where he gets his sister-in-law to invite Jessica to the party, and the third where he plays the mysterious stranger at the party and persuades her to ally with him.
This also gave me plenty of opportunity to deepen characterization. I got to show John as so obsessive about his quest, that he'll track down this woman and then blackmail his own beloved sister-in-law to arrange the meeting. I also got to show that Jessica might have been tricked at first, but is smart enough to figure that out, and exact a few concessions from him.
Think of a coincidence in your plot as the opportunity to go deeper into your plot and character. Consider:
1. If you could not "just happen" to get this to happen, what would be needed to make it happen? If you need him to get somewhere to meet someone, why might he choose to go there?
2. How can you use this "causation" to show something deeper about this character?
3. Look to see if you need to go back several scenes to set this up, or to find a way to subtly explain to the reader how this happened.