To keep crabgrass at bay, it’s imperative that you keep your lawn as healthy as possible. Water it regularly, fertilize it seasonally (at least for 2 or 3 years). A nice thick lawn won't let crabgrass invade.
If you see a clump of crabgrass, pull it out! Do it early, before it drops seeds, then fill in that empty spot with fast-growing lawn seed....but not if you treated the spot with an herbicide. In fact, it's best not to do that. Just drop a handful of lawn seeds there, spread 'em out a bit, and give 'em some water.
Water that spot every day for a couple weeks or until you see lots of sprouts. Then water every few days, then a few times a week, then once or twice a week. You want the baby grass to develop a lot of roots at first, but then you want the roots to get strong and that's why you water less and less....gotta make 'em go looking for the water.
