In the same way, you should always make sure your plotlines aren't predictable.
When people (whether your mother, your critique partner, your agent, or anyone really) can read a few pages of your book and think to themselves, "I know how that one's going to turn out," it's time to shake things up. Zig instead of zag. Introduce an unexpected character vulnerability/situation/response. Take a "cliched" plotline and turn it on its head.
In other words, keep your readers guessing. And because they can't predict what will happen next, they'll stay interested and invested right to the end of your book.
How about you: Do you have any strategies for making your plotlines unpredictable? And how important do you think this is?
ETA: And check out Cally Jackson's comment below about the importance of any plot twists being believable. So true - shake things up, but make sure any twist falls within the realm of probability (1) in the manuscript's world, (2) with regards to the characters in your manuscript, and (3) realistically taking into account things the reader will believe possible.