K.E.
I had to use the playtex straw-style sippy cups at first with my son. Apparently, you need to suck pretty h*** o* a bottle and when he would suck that h*** o* a standard sippy, he'd get too much and choke and then refused to try again. With the straw cup, things went more smoothly and we gradually worked in the Nuby and then the regular sippy cups.
My crazy doctor says sippy cups aren't necessary, that they are just for parents' convenience. She says try a standard cup with a tiny bit of water to start and you'll be amazed at how fast they progress. (No, I have not done this, but thought I would pass on the "professional" opinion.)
Good luck,
K.