Your location, soil and surrounding weeds all contribute to infestations of weeds in the lawn. The best offence is a good defense so making sure your lawn is healthy by following turf management in your area is the best advice. I am not sure where you live so suggest you contact your local Master Gardener or better yet a neighbor with a weed free lawn and see what recommendations for the timing of herbicides and fertilizers in your area. Using a pre-emergent before the seeds of weeds sprout is effective only if the timing is right.  Remember that your landscape is more than just weeds in the lawn. Try to notice the other beauty in your garden as well so you don't stress about a less than perfect lawn. Keep growing, Marianne Binetti