Where do you live?

What kind of grass do you have?

Would you say the grass was dense? Can you look straight down and see lots of soil between the grass plants?

How often do you water? and for how long?

How high/low do you mow?

Believe it or not, all these are elements of weed control.

Dchall will tell you what weed and feeds are good for. I love his definition.

Look for Dimension 0-0-7. You will find it at many Feed and Seed type stores (mom & pop) or maybe a local garden center.

I Bought my dimension 007 at Lesco. It was $18 for a 50lb bag.

My lawn is only one year old. New construction. I did a mix of organic and synthetic ferts last year. Over seeded in the fall. Last summer was bad with crabgrass. It seems to have filled in well since early fall. I went and got some of the scotts halts.
The soil here is very sandy, I was watering twice a week, sometimes 3, because of the new seed and trying to establish. I will monitor it closer this year, hopefully it won't need as much water.