I use Verdone once a year on my lawn, don't know if it's the best, but I've certainly got a weed free lawn, despite the fact that my house is surrounded by fields full of Dandelions.
I endorse Heathfield's suggestion about salt. I can't remember where I read it but I used to do it although i didn't use boiling water. I think it's probably better to dig them out completely though- roots and all