Using my thumb like a pick (double thumbing) was probably the hardest thing for me to get used to and get good at. My advice would be just a lot of practice. It's really a matter of just getting the "feel" of it. Make sure you don't dig to deep on the up stroke, just enough to let the string catch your thumb. Also remeber to stay relaxed and play light, there's no need to over do it.
What helped me was to play scales using double thumbing, try eigth and sixteenth note feels. As with anything new start slow and gradually increase speed as it get's more comfortable.
good luck!
