The gist of playing Janna is simple. Push, push, push, and then support late game. This is the build I use most with her, although, some of the items are situational. With the update, I really like the Ionian boots of lucidity for even greater cooldowns so you can pop your shield on your allies giving a healthy 622 HP in a teamfight roughly every 5 seconds at full build. Even then, the cooldowns really kick in once you get your soul shroud, so casting often shouldnt be a problem.

The aim of the game is NOT to nuke. Her gale is not reliable enough and her only other offensive spell doesnt hit hard enough or often enough to make you offensive. You are better off harassing enemies and covering lanes. Your speed should be more than enough to get where you need to be. When a lane is left alone, and you see 4 or 5 of your enemies covering a lane, teleport to one of their empty lanes and steal a tower. Use your shield and get some extra damage with each hit.

The runes are set up so that you dont need to worry about mana at all late game. Plus, your mana/5 seconds really ramps well as you level. Around level 6, you should be able to cast fairly frequently.

The armor pen works well at obliterating creep waves early, and helps with late game pushes or defence.

Lastly the cooldowns are just nice :)
That is all xD
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Mastery wise, Fairly straightforward caster utility picks, and the spell penetration and cooldowns from offence. Archaic knowledge is absolutely a must. You MUST pick it.

I focus mainly on supporting a carry, and setting up kills. Its easy to try and hit the enemy with your gale, but unless you have a good grasp on predicting where they will go, dont use your gale on them. Instead, always focus their minions. You can use it to help feed your carry last hits, or activating it early to knock a little damage off your enemies if you can spare the mana. Most often you will be targeted if you hang too close, but a clever maneuver is to fire your howling gale, activate your shield on yourself and target the caster minions. If you hit each one before your gale channels, your gale will net you last hits, not to mention being an efficient method of pushing.
Watch for champs like rammus or shen though, as you are horridly squishy and will get taunted to hell. TF likes to warp in and ruin your day as well, so be weary of bushes and have your shield and gale ready for a quick escape.

The first time you head back to base, buy your boots, either Boots of swiftness or Ionian boots if you like the Cooldown it gives. Really, depends on your playstyle.
After that, if you have been laning well, or long enough, upgrade your meki pendant to your tear of the goddess. Before you leave, dont forget to use your shield and gale AT the spawn point. its a fun way to gain a cost free mana increase from your tear.

This will be a combination of teamfighting and laning on your part. watching for your tank to initiate is an important function of janna. Applying your shield and immediately channling your gale will often be enough to keep your tank alive, and disrupt movement. remember, no one likes AOE stuns, and the knock in the air is more than enough to screw over any team. Getting your tear and shroud after your shoes should be top priority. Once you've saved up enough, get a will of the ancients. The spell vamp is amazing, and the AP boost for your team really, REALLY helps.
The Rabadons caps are for the AP boost to your shield. Jax will feel like he belongs on the short bus when you negate his leap + empower combo, and it is nice when DoT skills fail to knock off that last 100 HP on your otherwise screwed kassadin.
Lastly your ult is to be used MAINLY to knock back enemies from fleeing allies. This is what pisses people off, when they have 2 of your carries running at 10% HP, and you scoot in and push their entire gank back. Couple this with your gale to knock them up once more and you have effectively saved your allies :)

Either way. This is my first guide, and this build has served me really well. In a losing game, its still easy to avoid deaths, so play smart, and have fun!
(criticisms?)