A.S.
this might sound weird but I got a smoothie from Maui Wowi (or from Jamba Juice) and it was thick enough that when I drank from the straw some of the drink stayed in the straw and slowly went back in the cup. I took a drink and my daughter wanted to try it so badly. So I let her try. because it was so close to the tip of the straw her first suck she got a taste of it just kept sucking. I had been trying to teach her to drink from a straw for at least a month. From that point on she was like an old pro. Its hard for them I think because they are so used to sucking and getting a drink instantly where with a straw it takes a few sucks!
good luck!!!!