In the spring when crabgrass and other weed seeds begin to germinate, aeration is the best thing to get done followed by so called weed and feed product which combine fertilizer with a pre-emergent herbicide that keep weed seeds from germinating.

There's also an organic alternative called corn gluten, which like the chemical herbicides keeps crabgrass seeds from coming to life. As the gluten decompose, it adds nitrogen to the soil.