i spent two and a half hours pulling crabgrass today. my fingers are sore!
squzie, the only way to get rid of crabgrass once it takes hold is to pull it, with roots and all as jeeks mentioned.
then, reseed the holes it leaves and any other bare spots. ditto-ing jeeks again, grow a thick lawn so crabgrass has nowhere to grow.
next spring, be sure to put down the fertilizer with halts in march or april, to prevent weeds from being able to germinate. you have to seed in the fall, before putting that stuff down, because it will also prevent the grass seed from germinating.
i ripped out my entire front lawn last fall and reseeded it. i didn't get the early fertilizer down, and my neighbor had a yard full of weeds, which of course, grew in every tiny bare spot in my front lawn where the grass didn't grow well.
fortunately, the neighbor sodded his property a month ago, so i should be able to get control of my own lawn now.
tomorrow, i'm thatching with a rake, getting some soil spread out to fill holes, and seeding my entire lawn, again.