The easiest way to stop crabgrass is to nail it before it starts growing. Apply a preemergent herbicide after your second mowing in the spring. Crabgrass grows from seeds scattered in previous years. The herbicide keeps those seeds from germinating. The seeds may remain viable for several years, so it's best to apply herbicide every spring. One springtime application will vastly reduce the need to attack crabgrass later in the year once it has sprouted.
Here are more tips for getting rid of crabgrass.