I add about a teaspoon of metasulphuron to my round up which I pre mix in 20 litre drums. It's a granular herbicide and it gives it an extra kick. If I do it right, it will kill everything down to bare dirt. With some of the jobs that I have done for years I find that I don't poison as much as I used to.
I have heard that if you put a splash of vinegar in the water it lowers the PH and makes it more effective, I tried it for a while but it didn't seem to make much difference.