What matters to your character the most? What would your character die for? Establishing personal stakes for a character is discovering what makes them tick. It answers the readers' question, Why should I care about this character for 400 pages?
Every great story has two things: great characters and conflict. Both internal and external conflicts are needed to progress your plot. But, you also need to think about the what if scenarios. And that is how you identify your character's stakes.
Now, take those stakes and escalate them. Turn them into calamities, natural disasters, and time bombs. Put more at risk for your character. Make them squirm a bit, make them suffer, and definitely put more on the line.
To escalate the stakes, you must make the reader feel like there is more to lose. Put your characters in a situation where they have to fight to survive. Answer the question: How could things get worse?
Try these prompts:
1) Who is the ally your main character cannot afford to lose? (KILL THAT CHARACTER)
2) What is your main character's best physical asset? (TAKE IT AWAY)
3) How much time does your main character have to solve the problem? (SHORTEN IT)
4) What does your character cherish the most? (DESTROY IT)
That is escalating the stakes! Though it seems terrible, in a way, to torture the characters we love the most, it provides the most drama and conflict. The more you push your character toward the edge, the closer you will pull your reader in. It's the law of force and motion.
There are a lot of examples of escalating the stakes in YA. Look through your manuscripts or books you have read recently. Where does the author escalate the stakes?