I've just recently started taking the time to practice a lot of slap stuff and to me this is one of the hardest things to get. When playing fingerstyle I constantly anchored my thumb so it's quite an adjustment for me as well. What I've found works, for me, is to play closer to base of the neck, above the pickups and use my wrist as an anchor right over the pickups. It seems to be the best way to get a consistent sound and feel when DTing. You just have to remember that absolutely nothing is in stone and that you need to experiment with it until you find what works for you. My suggestions are just an attempt to give you ideas about stuff to try.
