When Buffy's mom is trapped in the wall, I take it as forshadowing of her death (there is obviously something disturbing to Buffy about her mom being in the wall, yet Joyce seems unconcerned).
I see the events in "Family" forshadowed by how Willow seeks comfort in Tara and yet is unable to find her or see her face much.
Riley and Adam being so similar and working together is forshadowing to me how Riley can't really adjust to the non military way of fighting evil.
I think Buffy's death is subtly hinted at when the First Slayer keeps telling her she is alone.
In Graduation Day Part 2 when Buffy and Faith are in each other's comas, Faith says "Miles to go, Little Miss Muffet counting down from seven three o" (it was seven hundred thirty days later that Buffy died).
When Giles sees Spike as a sort of celebrity, it seems to indicate that Spike will take his role as the patriarch of the Scooby gang.
Everyone can probably come up with a lot more than that.
