Assuming you don't get assaulted by a Yi, Vi, or Yasuo at the wrong time... The key to supporting is this: Come mid game, identify who on your team has the greatest destructive potential. Facilitate that person. Keep them alive, buff them, embolden them with flanking maneuvers. Whatever you do, keep _that person_ in the fight. This is where I've had the most success at support lately.
Edit: Some people seem to be getting a little sarcastic. While this isn't abnormal for the internet, I want to make clear what I mean here. I'm saying **ignore** the rest of your team. Every single one of them, no matter how well or poorly they're doing. Your ADC, your top, your jungler, I don't care if they're running to you with Ignite on them and you have a heal up. You save that heal for the top performer, unless they're so far away that your heal has time to cool down before you reach them. This is how I'm having the most success of late.