It's best to use fertilizers with some slow-release nitrogen such as ureaformaldehyde or sulfur coated urea. That way, you can make fewer applications, your turf will receive a steady source of nutrients over a longer period, and there's less chance of fertilizer burn. Stop fertilizing cool season grasses in early spring and warm season grasses in late summer as they begin to go dormant. Never mow your grass when it's wet, as this is one of the best ways to spread disease. Also, keep your mower blades sharp.
Dull blades rip the grass, stressing the plant and creating wounds that attract insects and disease.
And don't cut your grass too short. Longer blades shade and cool the plant's crown, where growth occurs and provide more surface area for photosynthesis. Also, think about investing in a mulching mower, which adds organic nutrients to the lawn. Most cool season grasses should be seeded from early to mid-September. They'll become established before hard freezes and avoid fall leaf drop while young.