Andre suggests setting your mower blades to cut the grass no lower than 3". Grass mowed high develops extensive root systems and will shade out and compete better with broadleaf lawn weeds.
Lawn mower blades should be sharpened several times during the season. When your mower blades are razor sharp, they slice off the grass cleanly. Dull blades tear and damage the grass blades leaving the lawn looking scorched and may lead to disease problems.
Deep, slow watering (putting down 1 1/2" water each time) every 7-10 days leads to healthy lawns with deep root systems.
Use organic fertilizers such as Espoma Organic Lawn Food or Milorganite to fertilize your lawn. These organics release their nutrients slowly over time so you only need to apply them twice a year - once in spring and again in fall.