At some point in the combat, just when the heroes seem to be getting the upper hand for the first time, more bad guys should arrive. And ideally they should present an escalating threat by being more competent and better armed.

This principle is evident in Star Wars from the very first combat of Episode I, where the low level mook battle droids give way to the arrival of the reinforcing droidekas. It's also used in Episode IV when Vader and his elite personal squadron of TIE fighters join the Death Star battle. No doubt you can find many other examples throughout the history of film and literature, and of course it's highly applicable to roleplaying games as well.

Transcript

Boba Fett: You're not taking that brain anywhere, Solo!! It has the information I need!
Chewbacca: If it's information you want, his might not be the brain you're looking for.
[SFX]: whooooshh!!!... {Boba jetpacks over from the sailbarge to the skiff}
Boba Fett: I must find Obi-Wan Kenobi!
GM: Boba waves his blaster at the party.
Luke: Disarm! 16!
GM: What, exactly, did you mean by "disarm"?
Luke: Slice his blaster!
[SFX]: Whooom!
[SFX]: Sliiiice!
GM: Done!
Chewbacca: Quite handily done, Luke!
Boba Fett: Fool!! You should have chopped off my hand while you had the chance!
Luke: There are enough small arms around here already.