I use Ortho's Weed-B-Gon and it is very effective. Will not harm the grass but gets weeds other than dandelions, too. It costs me about $14/gallon, but I have no idea what it is up your way. As others mentioned, thick, tall grass is key. I mow high compared to most folks, but it discourages the weeds and looks quite lush.
Every day I go out to spray those two or three dandelions that have sprout their pretty posey, I ask myself if it's worth it since some of the neightbors have thos yellow carpets around their houses?!??!