When PCs are busy being shocked by one shocking development, throw an even more shocking development at them.

It sounds simple enough, but this sort of planning can help make an adventure truly memorable. Uncovering a huge and startling revelation feels better if there's a lead-up of smaller discoveries and deductions along the way.

If the town burgomeister who hired the adventurers to save his daughter is actually the vampire himself, don't reveal it too early. Make the players think that the twist in the straightforward vampire adventure is that there's also a werewolf involved, by dropping some hints leading in that direction. Once the players starts thinking "werewolf", ramp it up by having the werewolf be the handsome blacksmith who was courting the burgomeister's daughter. You could even toss out hints that maybe the daughter is infected and a werewolf herself. But they rescue her, find she's free of the curse, and return her to her grateful father.

And then they start finding hints that he's the vampire...

Transcript

Leia: Nooooo!
Darth Vader: I had your blood tested, Senator Princess Organa. DNA analysis. Midi-chlorian count.
Darth Vader: Princess isn't your real name, is it? Your name is Leia.
{beat}
Leia: How the hell do you know that? Only my parents know that! They told me never to tell anyone!
Darth Vader: The Polis Massa medical staff revealed certain information after some... persuasion.
Leia: Well, now you're just being funny. Ha ha.
Darth Vader: You're not the Organas' daughter.
Leia: Duh! I've known that since I was, like, six. I'm the proud daughter of two heroes who died... fighting the likes of you!
Darth Vader: No Leia, you are my daughter.
Leia: That's not true. It's impossible!
Darth Vader: Search your DNA report. I know it to be true.
Leia: No. Noooooo!!!!