Arrange for your character to finally meet up with someone who reveals the deep truth about who he really is. While all along the character believes he knows his past, someone tells him where he really came from.
Lead the reader in one direction towards the end as though you know just where the protagonist will end up for the rest of his life. Then throw in a surprise ending with a deadly illness that will change the entire course of your protagonist's life.
Make the protagonist someone the reader loves and knows well. Suddenly, toss in a twist giving the protagonist a horrible flashback that shows something about his childhood. Trickle in a few more flashbacks leading up to his life today and end the story.
Add a deeper perspective to the story. Make a surprise ending by having your protagonist, who's a spy for the government suddenly turn out to be an alien. Now, all the United States information becomes information for the aliens.
Change genres. Tell an exciting thrilling adventure story packed with action. As the story is just about to end bring a change by making it into a murder mystery. Be careful though not to manipulate and trick the reader too much.