James offered some good ideas here for the same question:
Do you know what caused the stain? If it was from leaves you can try a few things. A good stiff wire brush and elbow grease may remove the marks, Power washing also does wonders, but you need to be careful if they are soft as the power washing can eat away the surface quite quickly. As Steve said in another post acid wash is also one way to do it, but It in my opinion the last option as acid is hard to work with and can cause burns if not properly handled.
In any case, tell us what you think stained the cement so we may be better able to help you out.
they are most likely rust stains....there when i moved in.
Then a wire bush should really do the trick for you.