"Whistle while you work"
Spike's line "Someday he'll be a real boy" is a "Pinocchio" reference.
Anya's line "Look at me, I'm dancing" is also said by Jerry the mouse in his dance number with Gene Kelly.
After singing "I'll Never Tell", Xander and Anya collapse on a couch and start laughing hysterically, just like the main trio in "Singin' in the Rain" does after the song "Good Morning".
Spike's line about "seventy-six bloody trombones" is a music man reference.
The line "That's entertainment!"
Mentioned in dialogue
Xander's line: "Respect the cruller. And tame the donut!" In Magnolia, Tom Cruise says "Respect the cock! And tame the c*nt!"