Midpoint twist are always fun to write, but it takes some tough brainstorming to figure an option that still flows with the story and is surprising to the reader.
You want to be constantly switching between good things and bad things. This really applies to your whole novel, but you have to make sure it fits in with the plot twist. Even if the good things are small, the reader will stay more interested if you switch off.
Give your readers something they wouldn't expect. As I have already said many times, making a novel unpredictable will keep the reader into your story. But, creating a good midpoint twist is not always easy. In my book, something bad happens to my protagonist (I am not sharing the details, because I don't want to give it away), then something good happens. But this thing that all the readers thought was good is actually really bad.
The hardest part is coming up with your twist. I usually like to invent it when I am in my planning mode - suspensful music playing, brain thinking faster than my hands can type. But, if you don't have time to wait for that wonderful and inspirational moment, then what I suggest is do some research and read other book's midpoint twist (probably a book that you have already read).
Think about what happens after the midpoint twist. After your midpoint twist, usually things seem hopeless for your protagonist. The antagonist seems to think of every option the protagonist has to win and blocks it. The midpoint twist is usually the start of your novel's "turn for the worst". Think about the best way to start what comes ahead. Will your protagonist be captured and beaten? Or will he/she be hindered in some other way?
By the way, I probably won't be posting a lot during the month of April because of Camp NaNoWriMo. But whenever I get the chance, I will definitely post! :)