Planting lawn seed wherever your lawn is thin or bare will get rid of crabgrass.
Take a look at http://www.grassstitcher.com/ to find out about the new revolutionary device which gets this job completed efficiently. Lots of people apply pesticides every year so they can avert this nuisance weed, and then become dissatisfied. If you're able to see the soil, you can bet you will probably end up getting crabgrass later in the summer. The key is to shade the soil from the sun's rays by planting lawn seed through these patchy areas.