To add to the examples presented in the first answer, I would point to the title itself - Death of a Salesman. We find out that Willy's sales idol Dave Singleman embodies the concept of the almost regal death of a salesman with admiring mourners from all over the world. However, the audience soon understands that the title does not only foreshadow Willy's death, but a death that in no way resembles Willy's fantasy.
The second is the small rubber tube that has "a little nipple on the gas pipe." This indicates to Biff as it does to Linda that Willy is not only considering suicide, but taking steps to act on it as well. Again, this seems to be Willy's recourse when his reality and his fantasy do not match up.
A third example of foreshadowing would be the constant reappearances of Ben, a representation of a more hopeful time for Willy. He constantly asks "Why didn't I go with Ben to Alaska?" He feels more and more like a failure which leads him into less and less contact with reality.
Three examples of forehadowing in Death of a Salesman will suffice: The first emerges before the beginning of the play, where Willy Loman's car accident presages his suicide at the end of Act II. In the second, Willy's angry fixation with Linda's nylon stockings repair foreshadows the revelation of his affair with The Woman, a secretary to the buyers, to whom he bought access by the gift of "mama's stockings". Finally, in the third, an aural one, the flute theme which we hear at the outset of the play betokens the revelation of his father's occupation as a flutemaker and his abandonment of the child Willy.