Weed control is typically a two step process, prevention and elimination. To prevent weeds apply a pre-emergent herbicide before it rains, if rain isn't in the forecast make sure to water the product in thoroughly after application. To kill existing weeds apply a post-emergent herbicide. This can be done a day or two after a pre-emergent treatment, which can actually speed up the process, because the weeds should be nice and green after being watered. Healthy green weeds absorb herbicides and die quicker than unhealthy weeds.