I would echo that modeling the behavior is helpful. Sometimes, though, I think they either get the straw, or they don't.
DD1 did not understand the concept behind a straw initially, her first sippy cups were the hard spout. DD2, conversely, did not understand the spouts (could not figure out she needed to tip it back to drink), and took to the straw at around 10 mo. Good thing about using the straw is like becj86 says above, you can go out anywhere and they will be able to drink without the need to schlep around a sippy.