There are two major things you have to do to control weeds. First, suppress them at the beginning of the growing season. In particular, you need to go after danelions and crabgrass as, once that stuff matures, it will grow over the top of your grass and smother it. Second, you need to get that stuff out of your ground before the cold weather arrives, because that's when most noxious weeds drop their seeds for next year.
If you can't dig up or kill dandelions, a simple measure of suppression is to pull off their yellow-flower stems. This will prevent those yellow tops from turning into gray fly-away seeds that make for more dandelions later.