This is something that's bugged me forever.
When you're reading along and something happens that makes you think, "Oh, Author, you totally wedged that in just because it's convenient to the direction you want the plot to go in. Lame!"
Don't get me wrong. We all do it. We all contrive events to shape the story. I've even discussed the joys of throwing wrenches into the works, just to mess with my characters. The problem is when the reader can tell that's what you're doing.
So, how to avoid? I think one key is consistency. If you get halfway through the rough draft and decide making Character X your MC's brother (plus he knew it all along, but kept it secret) is going to solve all your problems, great. But realize you're going to have to go back through and reshape Character X's early behavior. Not enough to give it completely away if it's a big twist, but enough that looking back, the reader can say, "Oh, yes, I see now!" (Foreshadowing/Hinting vs. Telegraphing ... have I done a post on that yet? No? Hmm, I probably should.)
When things come out of nowhere—even when there's nothing in the text to explicitly preclude them—it's just annoying. As a reader, it makes me feel like I'm being jerked around. I don't like that feeling.
What if the twist or turn comes in a later book in a series, though? What if earlier books are already published, thus establishing "canon"? That's trickier. I guess all you can do is try your best to make character and plot choices that are reasonably organic to what's already set in stone.
This is one of those things that I'm right on top of as a reader (and a hyper-critical one at that), but worry that I don't know how to avoid/spot/fix in my own writing. So if anyone has other thoughts or suggestions on how to prevent your readers from rolling their eyes, please—let's hear them!