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.