How to change a bad habit into a parentally-supported research project: turn it into a blog subject and call it a writing assignment. My teenagers love energy drinks and are always curious about the next one. Needless to say, as a Good Mommy, I disapprove (but don't ban them). Since they were always trying new drinks, they needed a way to keep track of the ones they had tried and whether they liked them. So they created a blog, the buzzed, and started writing reviews of the various brews. Then they just happened to let it slip in my presence. Once I saw that they were approaching the drinks seriously and doing some writing voluntarily, I started keeping my eyes open for new drinks and actually bringing them home. The boys appreciate it just enough to keep writing entries.
I like the fact that they are learning that writing has a real purpose, besides just getting schoolwork done, but I think the real lesson for them is in parental psychology!