From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156674-198969
Question: I have tried lots of different ways to kill crabgrass but it
still comes back in our ground cover, lawn, flowers, and vegetables.
What can I do?
Answer: This is the type of situation where an ounce of prevention is
worth several pounds of cure. It's easier to keep weeds and grasses
out of the garden than to get them out once they've established
themselves. The safest treatment is to dig the weeds and grass out.
Crabgrass is an annual that begins to sprout about the time the
forsythia blooms, so one of the best controls is to either cultivate
the soil lightly to disturb those seeds (lightly so you don't bring
more seeds up near the surface where they'll germinate, too) or to use
a layer of mulch to smother them. Another option is to use corn gluten
as an herbicide. The gluten releases an enzyme that kills germinating
seeds, and at the same time adds nitrogen to your soil.