As a green industry professional, I often laugh at the TV commercials and cable TV shows that are broadcasted to the public. The misinformation that is construed to the public is mostly false when it comes to showing pictures of lawns that are 100% weed free and perfectly healthy. Having a thick lawn in the first place is the best way to control weeds such as dandelions, clover, and henbit but, proper irrigation, fertilization, and ideal growing conditions are a necessity!
Have you ever wondered why from one year to another your lawn may look better or worse? If you relay on Mother Nature to water your lawn, this can be a big problem in the Summer, especially with the drought that we have had in our area the last two Summers. Air temperatures warm the soil and make the lawn and any weeds thrive. A neighbor mowing his lawn and blowing his lawn clippings into yours may allow for dandelions to germinate. Birds dropping seeds, while flying overhead, can germinate weed seeds in your lawn as well.
As a green industry professional, another issue that I see is simply cutting your lawn too short. This will encourage weed growth. Lastly, ever wonder why dead spots next to sidewalks and roads are usually dead in the Summer? The Summer heat warms up the asphalt and concrete and literally burns out the grass. While having high expectations is good, remember to think back to reality!