There are two significant references to the character of Dawn before she first appeared in Buffy vs Dracula:
In This Year’s Girl, Buffy and Faith are making a bed in Faith’s dream. They have the following conversation:

Buffy: “I wish I could stay, but…”
Faith: “Oh, you have to go.”
Buffy: “That’s just what…”
Faith: “Little sis coming. I know.”
Buffy: “So much to do before she gets here.”

As well as referencing Dawn verbally, the two Slayers are making the bed in Dawn’s future room.
The second reference occurs in the episode Restless. Tara says to Buffy, “Be back before Dawn”.
There is also a couple more, slightly cryptic, references: In Graduation Day (Part 1), Buffy walks into Faith’s apartment and Faith says, “Look at you, all dressed up in big sister’s clothes.” In Restless. In Giles’ dream, Olivia is pregnant, which may signify the coming of Dawn as Giles became a father figure to her after Joyce died.
In Graduation Day (Part 2), Buffy dreams that she and Faith are making a bed together (again!). Faith says, “Miles to go. Little Miss Muffet counting down from 7-3-0″. Faith’s riddle is explained later in the series. “730″ is a forshadowing of Buffy’s own death (Buffy dies to save Dawn). It means 365 multiplied by 2, meaning in two year’s time. This date coincides with the episode The Gift where Buffy dies. The number crops up again in season four’s finale Restless, where Buffy sees a clock saying 7:30 and Tara tells her the clock is wrong. It is: there are no longer 730 days before Buffy’s death, there are 365.