The best advice from Write Great Fiction: Plot & Structure by James Scott Bell is that since readers are more aware of potential plot developments and build up expectations based on the many repetitions of plot 'twists' they've seen before, you should fool them by refusing to do the expected. Basically, conceive of your plot, scene, twist, etc. and then put down the first thing that comes to your mind. The first thing is likely to be something done before. After all, you're also a reader and thus you anticipate of your own plot lines the same hackneyed tales that others do. So make a list of alternatives to your original idea. Brainstorm.
So, let's give this a go:
In the Medieval Era, a young woman who has always been an eccentric dreamer is told that she must marry a man twice her age. How does she react?
She secretly plots her way out of it through a mixture of cunning and wit.
She rails against it and stamps her pretty little feet.
She accepts it with a whimsical smile. Older men are less likely to be demanding of her time.
She has an affair to prove her independence and to cause him to break off the wedding date.
She joins a nunnery.
She pretends to be a boy and joins the army.
True, none of these are necessarily bad. They're all good reactions. However, you've got to admit that you saw 1. and 2. coming. 3. would be a surprise though it does mean there's no conflict in this piece of the plot. 4. is certainly unexpected but it'd be hard to respect such an act unless it was truly born of desperation as it would probably cause more trouble than it was worth. 5. is also unexpected and rarely used, fits the character concept (dreamy and quiet could make for a devoutly religious figure) and flouts societal expectations in what is still a socially acceptable manner. 6. would need a lot of build up and character development for that particular character but could be worked into the concept if her daydreams had a decidedly military bent.
And that's just what I get when I brainstorm 6 ideas. Imagine if I brainstormed 20. What do you guys think? Try the exercise yourself and tell me what you find.