I had the same problem last year. I went to several garden shops to
find if there was anything I could do. The best advice is that there
really is little that you can do. I ripped up chunks of my lawn and re
planted with seeds. There seems to be little else for simple answers.
However I can say that in the spring I did put down pre emergent seed
killer and it really worked well. I was surprised at how little came
back. So you may have to be patient and wait. On the plus side I bought
my pre emergent killer in the middle of the summer when it was about
half the price, as the garden centers where trying to get rid of it. I
held on to in till the following spring. I think that one of the things
that causes the crab grass to take over is the dry times in the summer
when I foolshly listened to the city and seldom watered. This gave the
crab grass on oportunity. I will not make that mistake this year. I
think a healty lawn keeps it at bay.

regards
iiiiiiiiiiiiii - picture of my grass.