UPDATED!!! to v.3 - 4/03/12 This update covers new runes and items! Build 1 is pure tank support, build 2 is support nuke.

The most important stats for Karma, are health, mana regeneration, and cooldown reduction. The trick is to find a way to get these stats, while both supporting your team and getting a moderate amount of ability power.

Treat Karma as a caster support version of Olaf.

This guide can help you play ranked better.

The key to playing Karma is knowing which skills to use when.
You have 2 mantra charges max, therefore you have to pick which skills to amplify, and when. Karma pretty much has 2 styles of play, offensively defensive, and defensively offensive.

If you are solo defending a turret and someone tries to jump you before you can get under your turret and/or turret dive you, you need to be offensively defensive. What that means, is that you mantra charge your bubble to prevent further incoming damage while doing some. Then mantra charge a heal to recover some of the life lost before the shield, while doing some more damage. Leash the target who chose to go too deep so the turret kills them. Use your summoner spell heal in emergencies. Also, you may choose to mantra charge your leash instead of your bubble, this works also.

If you are assisting another play on a chase, or trying to win a team fight, play defensively offensive. This means you either mantra charge a leash on your carry, making them your pet. You may choose to mantra charge the bubble you are going to throw on them to help them snag the kill, or you can save it for a heal if they need (in team fight always use the heal). Always leash and bubble an ally diving for a kill. As you scroll down, you can see why I have chosen the Reverie and the Rylai's. You can both slow the enemy with the mantra charged bubble and use the Reverie active for more speed for your ally or allies present.

Like with any support, let your gold per 10 items help you buy wards for your team.

Shut Up and Buy Wards Billy!

As seen on LoL Machinima by wowcrendor

The build listed shows my core items of choice. They can be changed, and here are some examples:

Your primary item you always have to have is:

Start with a Mana Manipulator, and follow with a Philosopher stone to provide your self good sustainability as a support in game, and the gold generation is nice. This Philo can be sold back later depending on enemy team composition. Also grab a heart of gold which can also be sold back for the same reason.

First 3 items in order will be:

The boot I prefer for movement speed purposes is:

Depending on enemy team composition, this can be replaced with:

Or

From here, you have to really pay attention to their team composition, you build to counter them, let your runes and passive handle your ability power.

If the enemy team is building primarily AD grab:

and

Late game make your:

If the enemy team is building primarily AP grab:

and
If their team comp is primarily AP heavy, but has a champion like Ashe, who can hit physically hard endgame, I would just ignore her. Her role is to stay at the back, as is yours, so you shouldn't get yourself caught in melee range of her. If you do, you have your leash to get yourself away, as well as a bubble and a heal. Karma is a great counter to melee champions like Yi and Trynd because of the leash. Put a leash on it! :P

By now, you have 4 or 5 items depending on whether the enemy team is AD or AP heavy. Typically in ranked you don't have to worry too much about AD, the jungle ganker is counter-able and his gank survivable with your combo of leash bubble and shield.

If you didn't buy:

Your suitable last item is:

orororor

You can also grab a second one of these in place of:

If you did buy:

Your suitable last item is:

orororororor

Update!

As the new item was released, the Locket of the Iron Solari, I have chosen this an a last item, or option replacement for Randuin's Omen based on enemy team comp. Great way to further counter heavy AD teams and live along with keeping your team alive!

For late game team fights, if you didn't get the Locket, I really recommend:

This is because your leash (Spirit Bond) does damage to any enemies that the leash contacts other than your target. This means that if you can snag a leash on someone toward the back of their team during the fight, you can do minor damage to most of them, applying it's slow, plus the slow of your Rylai's.

The other key factor here, is if you have a Randuin's and a Reverie, you can hit both actives at the same time, further crippling the enemy team while providing your team with a much needed buff.

Shield your squishy, heal the team, and above all, put a leash on that carry!

Some proof it works:

My 4 games prior were also wins, believe it or not. Yes, the best score was in a ranked game, it seems more efficient in those do to difference in play style, at least in higher elos. Nothing can save low elo.