Spray something with 2-4D like Weed-B-Gon, when there is at least a day of no rain that follows, twice a year (May and Sept).
I spray with a 30 gal tank and 12v pump, and a 6 ft boom. I mix 1.5 qt of Weed-B-Gon and 1.5 qt of Amine 400 to 30 gal of water. I get very good results on dandelions, thistles, creeping charlie (that is a tough one), and dock. Also keeps the clover in check.
Recently I've heard it's best to spray when the weather is warm (80°) but a week and half ago I sprayed when the temp was 60+° and threatening rain, and had a great kill on the dandelions. If you see them curl within 24 hours, you likely got em.
I find a fall spraying really helps keep the dandelions in check, as there are very few to spray in the spring. But not keeping up with the program of twice a year, will give them a good foothold that takes a while to get back to nice grass and no weeds. I spray about 8 acres.