Generally speaking anytime you deal with a weed you should "drop some seed". Thick turf is the best weed prevention going. Assuming your weather is fairly warm both vinegar and herbicidal soap work very well with crabgrass. You can use the white vinegar in your cupboard if you like or buy a product called Burnout which works a little faster. Soaps are available here in IL at Home Depot and Lowes so I assume they're marketed nationally.

The single most bestest way to prevent crabgrass growth is to grow a good, thick, healthy lawn so the crabgrass seeds cannot germinate, or is forced to grow more upright than is normal and that starts with a good, reliable soil test to see what your soils pH and nutrient load is. Contact yor local office of the University of California USDA Cooperative Extension Service about having that done. Then dig in with these simple soil tests to see what you have for soil,
1) Structure. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. A good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drains too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer you soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.
and what you need to do to make that soil into the good, healthy soil turf grass needs to grow well.
You might also ask the agent you talk with about which crabgrass, annual or perennial, you have if not both.

Here is a link that might be useful: UCAN CES

IMHO and experience, the only organic cure for crab grass is a long rake and lots of Elbow Grease accompanied by a large plastic garbage bag
If anyone has proof otherwise, I will gladly try it.

In terms of "green" lawncare, I agree that manually pulling weeds until you maximize the soil content is the best way to go. Healthy turf will establish root networks and above ground growth that will discourage weed growth.

Here's an article about organic herbicides and fertilizers; hopefully, it can help you get a handle on that crabgrass!

Here is a link that might be useful: Organic Herbicides and Fertilizers

yeah, my sympathies to the OP. I've got a lawn full of crab. There's one section, about 1000 sq ft, that was almost entirely crab. Don't know if this was a good thing or not, but out of frustration, I dropped my mower blade as low as it would go and scrapped the soil. Of course, I bagged the cuttings and composted. I figured, that I at least got rid of a bazillion seeds!, I then raked and pulled, and scrapped out what I could and dropped down seed. I'm going to hit that area real hard this winter with dormant seeding in hopes that tuffgrass will beat the crab germinating. For the rest of the yard, I've been hand pulling after every rain (and we've had some good ones this year) and dropping seed. It's alot of effort, but it does look like it's paying off.

Thanks to everyone for their input on this crabby subject. :)

-Ray