I think must haves for a nice lawn are:
1. Spinkler system (not a must for smaller yards, but it makes it much easier and they never forget or are too lazy to water)
2. Constant attention to weeds- I manually remove as much as I can and spray the entire lawn three times a year
3. Constantly patching dead spots from pets or whatever
4. De-thatch 1 to 2 times a year.
5. Sharp mower blades, level deck and constant mowing (careful not to wait too long because it can harm the grass if you are cutting off a large percentage of the plant at once)
6. Fertilize often, even before winter if possible (but be careful not to burn the lawn-ask me how I know)
I recently switched to a liquid fertilizer because it ends up being about 1/3 of the cost I was paying for the other stuff. I get mine from fleet farm 2.5 gallons will do 50k sqft of lawn.