I've been with Greenskeeper for four years and they do year-round service on my lawn. I pay them in the beginning of the year and they come once a month to treat my lawn and I'm happy to say that I haven't had one weed since.
It does take awhile to get your lawn looking how you want it, but after a few months you can really see the results and our lawn has never looked better. They do a really great job and their service is excellent, I highly recommend them!