I don't know if I am "high ELO" I am plat V right now.
I think something that you might not think about too much yet (maybe you do, idk) is pairing with your ADC. For example, you don't want to pick Blitz if your ADC is Kog kinda thing. So, I don't know all the best match ups, I normally just think about "Will they want to farm? Or fight?" And then try to pick a support that has a kit either based around locking the enemy down, or protecting my ADC. (Personally, I'm not a fan of the APC-support)
Normally in solo queue champ select doesn't matter very much. But, as you get to higher ELO it does start making more and more of an impact. I honestly believe this start with the support. So, even if you and your team are forming a full "siege comp", or "protect the ADC" comp you as the support should be putting in a little extra effort in your picks.
As mentioned I normally play champs I can either focus on locking down the enemy, or protecting my ADC. This applies after laning phase too. Don't think that in a teamfight you suddenly become a top or mid laner. You will not have a high-damage build (unless you are an APC-support, which again, I don't do, ever). So, there is no point in you trying to help do damage. You want to get your CC out as much as possible and you want to keep your carries alive. Don't go chasing the low health Ahri and leave your carries exposed. I know, easier said than done, sometimes I can still make similar mistakes in teamfights, but really try your best to stay focused on the protect/CC aspect of the teamfight. You ADC will be trying to do damage, but they need to be alive to do that.
A little on specific champ play, if you don't already know:
In lane as thresh it is easier to land CC if you get close enough to flay them towards you first and immediately follow that with a hook.
As Morgana, you don't always want to Black Shield your ADC, an easy counter to tht kind of play is to focus the Morgana instead. So, wait until the CC is thrown (any kind of distance CC) to shield the person being attacked, or if the walk right up to someone, shield the person with people on them.
You might be surprised how strong your poke is when you E yourself and hit the enemy. Ideally you E your ADC when they attack, but if your ADC is farming you can take it upon yourself to do some damage.
The other thing, is that while some champs and some circumstances would make you prefer Merc treads, Boot of Lucidity, or Ninja Tabi, Mobility boots are an amazing go-to on pretty much everyone.
I understand that APC-supports can work, but it requires you to catch enemy messing up in lane and it means you have to steal kills from your ADC. (taking a kill isn't bad, but when you can you should let the ADC have the last hit.) So, what you end up being in mid game is a roaming kill machine. You leave your ADC, alone, and just go take kills from other lanes. Again, I know this can be "done right" and can work. But, I personally don't think it is all that good.
and 
(if that means anything).
