Generally, Circle is about mobility and the idea that if you get the first hit in, you don't need to worry about getting hit back. They have kind of a scalpel style, where they can't wipe out everything, but can apply focused damage to eliminate one thing that really needs to go.
Shifting Stones are the cornerstone (pun intended) of Circle strategy, and they're a tricky unit to use well, because you need to plan what you'll be doing with them next turn. But one good teleport can hand you the game if the enemy warcaster wanders too close and gets a faceful of Warpwolf. The Stonekeeper is well worth it, since he extends the range they can operate at (and the number of things they can threaten to teleport). Also, pKaya can make great use of Serenity--if you can put a beast near a Shifting Stone when you use Spirit Door, that lets you push it for one more Fury than you normally could without risking frenzy.
Druids are a good toolbox unit, though it can be hard to make them earn their points initially. A common use for them is "fishing", where they get staggered towards an enemy heavy that thought it was safe, then use Force Bolts to drag it into charge range of something big and scary. And of course, a line of clouds is good to deny shooting against whatever's behind them. Against a ranged army without many blasts, it could be an option to use Occultation on the druids, so they can't be shot, neither can anything behind their clouds. Remember that if a Druid gets killed, their cloud will remain in play for the rest of the round.
The Warpwolf Stalker is probably the best single beast in the faction, but it's very delicate if if runs into something that can reliably hit it. Berserking with pKaya's
MAT bonus is a good way of clearing infantry, and it's quite easy to keep it safe from retaliation between Spirit Door and it's animus.
As for what else I'd suggest:
For
raw damage dealing as you expand, I'd suggest either a second Stalker, or a Feral. Once you have that, the Gnarlhorn Satyr or Pureblood Warpwolf are both excellent second-line beasts, able to augment your heavy hitters with their animi, or fight reasonably well themselves. The Pureblood's ability to lend other Warpwolves Ghostly won't come up often, but will make a big difference when it does.
The Gorax is probably the best light beast in the faction. Apart from a backup Fury source and transfer target, it's also a pretty solid slugger in it's own right. But the best thing about it is the animus. Knowing when to use Primal is tricky, because you effectively lose the beast you put it on for next turn, but the bonus can make all the difference when you have something that needs to die right now, no matter the cost. Also, the Scarsfell Griffon is good with pKaya (still trying to make it work with other warlocks, though), giving her something that can work outside her normal control area, and pick off flanking enemy infantry or solos.
Tharn Bloodweavers and Woldstalkers are both solid infantry options that will sync well with nearly all warlocks. Bloodweavers are another versatile unit who can clear infantry, deliver heavy damage against beasts and heavy infantry, or remove buffs. Woldstalkers are a reliable shooting unit that can do tricky stuff with the Stoneward's magic--use Zephyr for 3" of extra threat, or get a free 3" move out of combat, then get aimed shots at whatever was engaging you.
Megalith is quite expensive, but well worth it for what it brings to the table. The animus helps make high-
DEF infantry manageable, it can heal unlike other construct warbeasts, and can punch pretty well, plus an extra spell a turn is never wasted.
Gallows Groves give you mobile arc nodes, as well as Tough denial. Something I've found good with PKaya is to stall one enemy heavy with an arced Spirit Fang (to hit it with a speed penalty and thus deny charging) while focusing on the other one.
The Druid WIlder is a good support model, especially if you're taking multiple beasts with animi that she can use, like the Gorax, Stalker, Gnarlhorn, etc.
Hope that helped! If you've got any other questions, go ahead and ask.

Some more references you might find handy:
http://privateerpressforums.com/showthread.php?11315-The-Wilding-Way-Tactical-tips-for-Druids-both-new-and-old
http://battlecollege.wikispaces.com/mkiiCircle