My lawn looks amazing and I do two simple things starting in April.
#1, Fertilize while the grass is still brown and just starting to green up. Use a fertilizer with a high first number.
#2 Two weeks later when the grass is starting, over seed with good quality grass seed. Two weeks later over seed again.
The result is a very thick lawn that the weeds have a difficult time penetrating. I still have to pull a few weeds every week before I mow, but not too many. My neighbours all say that I have the nicest lawn around and ask what my secret is. Pretty simple eh?
I only use Scotts products which I purchase at Costco. My lawn is so thick that I have to raise my lawnmower higher until the summer heat kicks in and the growing season slows down.