Vision!
Wards, tons of wards, every time you are back by shop, wards. Start with one and progressively build your set of items and then you can fill your inventory with wards if you want to. You mainly want the "Rod of Ages". With Clairvoyance and some deductive skills a la "How fast and how far can he have gotten since I last spotted him?" you give vision to your entire team. Ping so that the map gets attention. Sometimes you might need to write out that you are simply spotting (and that it is up to them if they dare to gank).
+Clairvoyance
+Wards
+Mobility
-Still a juicy steak on a silver plate
-Bad Clairvoyance's (Serving little purpose, it happens)

Which takes me to the second section.

Communication
Endorse people, both sides in fact. It doesn't hurt to compliment someone doing something cool, I always laugh if the other team simply crushes our team in team battles because their teamwork is da "boss". Do your SS's. Don't whine, be nice, Janna is a support so you need to act like a support as well. Call it "cheering" if you will.

Co-Jungle
I have personally never tried this myself, but it just hit me that a Janna with Shield can boost the Jungler's attack and therefore jungle faster. Simply stand far away with Janna so she doesn't steal any experience.
+Jungler levels up faster
-Janna doesn't level at all (which doesn't matter much anyways, the second you get boots of mobility and a maxed Zephyr you don't need much more)

Co-Jungle Strategy
Early Game Blue: Shield Jungler or the one with the highest AD to take down the Golem quicker for the Jungler, ward top and teleport to bot and ward river.
Early Game Red: Suggest to steal enemy Red (and ward it afterwards), then teleport to bot and ward river.

Mid-Game/Late-Game: Janna is mostly bot, so whenever the Jungler comes down to bot and there is time or a situation where you can leave lane, Shield so that the Jungler can move on faster.

Mobility Sir Click-A-Lot
Your role with this build is to travel as fast as possible across the entire map to dominate the FOV (Field of view). Janna can barely take any damage, so your maneuverability is the most important, stay as far back as possible to spot any back attacks as well to give your group something to retreat into, such as an ultimate that pushes away the opponents or a Howling Gale. Slow-down key chasers. Alistair might be able to stun, but many times he can't do much damage.

Howling Gale
Q is one your most important buttons. You want to press it quick in succession two times. But you also want to direct it where they least expect it, and throwing it in odd positions. Most Janna's are very docile and passive. You want to be in the enemy's face, Janna is great at harassing. Spotting with Clairvoyance is going to be a nuisance for the enemy team, saving your teammates at the last second, throwing Howling Gale's right and left, you want to be annoying so that you get focused. Janna is the finest plate of juicy steak and everyone just wants to feed on her, delivering a rejection by the way of a dish served most cold is going to tick them off.

You want to split the enemy team apart, with her ultimate you want to get in a position where you can simply push them all in different directions. Most preferably into the open arms of your teammates, this is a very difficult maneuver. Doing this puts Janna in an awkward position, where she goes down very quickly.

Use Safety! When baiting, you want to be seen but you don't want to be too close. Like a nymph teasing the all too many male warriors of League of Legends you allure them towards you, if they aren't following it just move to another one, Janna is quick to jump to the next guy.

Runes
Honestly, I haven't looked into it, my friend told me "Try this" and I did and it works for me, I can probably make it better but it really doesn't matter much to me personally.

Videos coming soon...

Also I am very new at writing or explaining build orders through this "Guide" format. So bear with me, I'll update it. Let's call this "Document ver. 1"

Feel free to give me pointers :)

Have a good day! :D
/Osvir