Always tie up your loose ends.

Or, if you're running the adventure, always make sure the plot frays enough to generate plenty of loose ends.

Practically, you can achieve this by keeping some antagonist NPCs in the shadows, so they don't interact directly with the PCs. The team of superhero PCs can thwart the diabolical Doctor Gloom's plans to take over the city of Gigalopolis, and even capture the evil Doctor himself, but make sure that the reason he wants to take over the city is related to his relationship with the even eviller Black-Panther-Woman, who the PCs have only ever encountered in hearsay before.

Doctor Gloom may be defeated, but you can bet the PCs will have their hands full with the aftermath.

Transcript

Padmé: For the record, my good character kind of sees your point about bringing peace to the Galaxy by killing kids or whatever.
Padmé: But being good, I have to be a bit shocked by it.
Padmé: But... I still love you... Anakin, and we have to stay true to that.
Anakin: You can be the silk to my steel. I can keep order, and you can ensure the people are looked after.
Anakin: Padmé, with your rebellion you can destroy the Chancellor.
Anakin: And together, we can rule the Galaxy as husband and wife.
Padmé: Cool!
Padmé: So are there any loose ends?
Anakin: I'm sorry?
Padmé: Mace Windu knew the Jedi never rebelled, and Nute Gunray knows what he knew. We need to deal with him.
Padmé: Sorry, you need to deal with him. I'm good. This conversation never happened.
{beat}
Anakin: Did you just become devious?
Padmé: A little.