Lazy man's overseeding....
I have a neighbor who used to have an awful lawn who now has a pretty good one. Something that I've noticed that he does is once every couple of months, he takes a bag of grass seed and tosses some wherever it looks thin. He doesn't water it, mow low, fertilize extra, or anything else. Just overseeds the patches. It seems to have kept up the density quite nicely, so I decided to try it, too.
I have to say I'm impressed. Of course, it costs more than just putting down the proper amount of seed the proper way--germination is lower, and seedling loss way higher--but it works well for repairing thin spots during the year, before normal overseeding time.
I'm a convert!
(BTW, my front lawn looks better than when we moved in, and the previous owner was a lawn fanatic! All it needed was basic TLC. Didn't realize how off DH's care really was...)