We started with a honey bear sippy cup that allows you to squeeze it so the water goes into their mouth. My sister specializes in food therapy and it is what they use. It didn't take long before DD figured it out and could transition to the regular straw sippy cups.
I taught both girls with a straw. I just put a straw in a cup with water, put my finger on the top and then pulled it out of the cup. Then I put it in DD's mouth and when she sucked, I lifted my finger up so she could get the water. We did that a couple of times (or more than a couple of times) and that seemed to help her understand the concept.
Re: sippy cupt with straw