Whatever method you decide to try, practice it really slowly at first, and repeat it over and over. Build up your speed as you feel it is getting automatic. It takes a certain amount of perfect practice runs before it gets into your muscle memory. This will take awhile, so don't expect miracle right away. Expect to work on it several days, with multiple practice sessions each day. Once you have it, you won't have to think about it. I roll the flesh part of my thumb into the strings, and that seems pretty natural.
–Lori