Seriously use the Scott's site for your area and follow it to the letter.
Priorities are proper timing of fertilizers, weed killers, and bug killers.
You also have to water the lawn as well, have a set schedule.
Installing a sprinkler system is key if you want to maintain watering with no hassle.
Once your lawn becomes healthy you just need to fertilize and water, with some insect control thrown in, for a lush green lawn.