Three Plot Tips:
1) Ask, what do my characters (or I) expect to happen now? Make something utterly different happen.
2) Ask, what was true in the beginning of my book? What was the status quo? How is that changing? What would challenge that more? What would turn it on its head?
3) Ask, what else is going on, underneath what is going on? What else might be revealed? What do I assume? How might what I (or my characters) assume be absolutely not true?