INTRODUCTION. The next few posts will be advice directed towards playing support in general. The reason I'm splitting them up is that I want to keep posts on the short side but don't want to omit anything important. I describe my playstyle, which is not the only playstyle, but in my opinion it's both the best and most fun playstyle, which is why I play it. Your goal is to carry your carry in lane and snowball your carry relative to the enemy carry.
(A quick note---I say 'carry' and not 'ADC'. Usually I use these two words interchangeably, but in this case it's deliberately not ADC; your carry can be a bruiser or a mage, and your job as support is still to snowball him relative to the enemy carry.)
With this mentality, your goal is to play as aggressively as possible. The 'as possible' is the caveat here; you sometimes can't step any farther forward than your ADC is if, for example, you're both very low or the jungler is sitting in your tribush. Your carry is sitting back and farming, but you're at full hp and holding 2 more pots than the enemy? Walk up and trade damage. Hopefully your carry will catch on and walk up with you, but if not then you just burned a larger percentage of the enemy's resources than you did of your own. Learn your limits. How many autoattacks can you take from MF as Sona when you're level 2 and at half hp with this runeset? You don't necessarily need to know the answer to this question, but you do need to know when the answer is "one more" or "I need to flash out now."
Learning your limits you have to do on your own---even if you could memorize a large chart of numbers, there's no way you'd be able to figure out all of the factors that lead to your answer in the middle of combat without missing out on a lot of micro. It's like learning juggling: You can beg people over and over to help you learn, but unless you practice by yourself for a very long time, you're not going to get anywhere.
That said, you can still learn a lot from watching streams, reading guides, and watching your own replays. This series of posts will offer advice both directly related to playing games and also to using other techniques to improving your play.
RUNES. If you want a list of runesets to run on particular champions, look up my lolking profile. For the most part, I run the runes named "Champion Name" on that champion. I run my Leona set on Taric and sometimes Zyra and Lulu. Instead of listing runes for you here, I'll talk a bit about the logic that you should use to decide what runes to run.
In general, anything that's "all tank" can never go wrong in lane. Supports' base damages tend to be high enough that they don't need additional damage, and that tends to come with CC so that the ADC can output enough damage. When you're positioning as aggressively as possible, you want to mitigate as much damage as you possibly can with your base stats. Since ideally you will be exchanging 1vs2 most of the time, increasing your defensive stats is way more effective.
In certain matchups, you may want to run offensive runes instead, if you are playing a champion that doesn't trade with autoattacks/being close to the enemy as well (examples include Zyra and Nidalee). And sometimes the massive utility of another stat is so huge that you want to trade out defense for that stat. In particular, I run scaling mana regen quints on Nami and lots of CDR on Soraka, since neither of these supports trades with autoattacks that well and both scale extremely well with the stats that I use instead.
When deciding what runes you want to run (this goes for any position, not just support), play a lot of games and ask yourself, "What stat did I need in order to do better?" If you die a lot at level 2, try flat hp. If you die a lot at level 6, try flat MR or armor. If you run out of mana in late-game teamfights, run scaling mana regen, etc.
MASTERIES. A lot of the same advice applies here. Getting at least 9 points in defense is fairly necessary. If you get the biscuit in utility, you can stack it on top of an HP pot for a huge amount of hp regen---this lets you take an early ignite and you can use this sudden regen spike to bait a lot of damage and cooldowns from the enemy botlane before flashing out (remember to exhaust before you flash!!) and giving your carry an easy kill or double. (If you do this, rejoin the fight as soon as you have regen'd enough health to be able to do so.) For these two reasons, I suggest always getting at least 9 points in both defense and utility. It's possible to go both through Block in defense and through Explorer and 50 starting gold in Utility, so I like to do this setup even though it sacrifices the exhaust mastery; however, if you are playing a hard-engage support you probably want the exhaust mastery at the expense of block.
Next time I'll talk about itemization, which will be slightly more champion-specific.