As someone who mostly plays against Janna, and only rarely as, what I notice most about Janna's is timing.
Whether it's shield timing, or having a good innate sense of how far a tornado will go for its current channel time, or when(and where) to ult during team fights. Or just plain reaction speed to tornado gap closers.
In lane, she has serious power troughs when shield is down. Pre-shielding is something you have to be very wary of. Once it goes down, you and your partner are missing a major tool to handle a trade. I'd really suggest trying not to shield until shots are in the air if at all possible. I know that I personally abuse bad shield timing constantly when up against Janna players, so I'd say that's the biggest thing in lane.
Other timing and positioning is practice and game sense.