Okay, here are just a few tidbits on what to do at the midpoint of your story. The midpoint is sometimes referred to as a reversal, or a big reveal, but whatever you call it, it should have two things.
First, an action or incident from which there is no going back. This is not one of those moments in the story where you can be wishy washy. As the author, you have to make a decision and make it stick.
Second, your protagonist should do some serious self evaluation. He (or she) has to ask some hard questions. What do I really want? What is at stake? Who am I? Can I win?
To help you out let's look at a few simple midpoint examples.
In our pretend story your protagonist works hard throughout the first half of the book to get the man of her dreams. In the middle of our story, when she finally nabs the dude:
A. She spends the night with him (we're talking sex here, not just a sleepover), but when she wakes up in the morning she has second thoughts. He calls out to her from the bedroom, he wants her to make him breakfast. It reminds her of her parent's relationship, a relationship that was all wrong for both her mother and her father. She asks herself; Am I like my mother? Do I really want this man?
B. She spends the night with him (S-E-X). He asks her to make him breakfast and it reminds her of how her mother used to cook breakfast for her father.
C. She takes him to her place and cooks for him. He seems demanding. He reminds her of her father, the way he used to boss around her mother. She asks herself; Am I like my mother? Do I really want this man?
Now, obviously, example A is your midpoint and it's not just because there is more writing up there.
It's because an action has occurred that she cannot undo (I'm pretty sure you can't UNsleep with people. You can say "nothing happened", but we all know it did:), and then she asks herself some questions. In essence, she holds a mirror (hello midpoint mirror) up to herself to ask what she really wants.
In example B, she spends the night with him, and is reminded of her mother and father's relationship, but she doesn't ask herself any hard questions. It's the questions, or more precisely, the answers to those questions that gives us the reversal we need at the midpoint.
In example C, you have the protagonist's realization that he is demanding like her father, and she asks herself the important questions, but there is no event that can't be undone. She is just dating him at this point and she can easily say, "Oops, my bad.".
Do you see how examples B and C could really make your midpoint drag? A reader might be saying to themselves, "Nothing's happening in this story." You don't want that anywhere in your novel, but especially not at the midpoint (or at your two major plot points).
Use your midpoint for your big scenes. Kill off characters, reveal secrets, blow shit up, but make sure that whatever happens it cannot be undone (You can't almost reveal the secret! Or reveal it and then take it back!), and it has to give the protagonist a reason to stop and re-evaluate what's been happening since page one.
Event that cannot be undone - Protagonist self evaluation - A change in the action of the story. That is your midpoint!!
Yes, there are a million different ways to do this. You don't need to have your protagonist sitting with her head in her hands thinking about feelings for two pages. You can set it up from the beginning of the story so that the reader knows that if our main character's mother dies she will be...homeless, helpless, charged with murder...whatever. When, at the midpoint, the mother does die we already know how our main character is going to feel about this. We already know it will change the direction of the story.
If you're muddling through the middle and you're stuck, try writing out a scene with the midpoint mirror in mind. If nothing else it will give you a better handle on who your main character is and what she wants.
If you wanna know about more than just midpoint, check out my post on Three Act Structure.
Also, for a look at my writing resources check out Books on Writing.