What I'm looking for are tips to know and skills to develop that distinguish intermediate players from beginners. I've won my first several Normal Blind games but I think I'm plateauing. In my last game we had several teamfight standoffs in mid where the huge enemy Cho'Gath would dance back and forth in front of his turret and various members of my team would watch him. I was often the one dancing around No Man's Land picking off minions, but I certainly wasn't going to initiate. I tried to be where the fights were and buy items that made me even more useful with auras, but I felt like apart from that and placing wards I couldn't push us over the hump (we eventually won because Brand was quite good on our team).
I think I understand the basics as Sona. I can rack up huge numbers of assists and even a decent amount of kills in bot games. I know what the abilities do and how Power Chord works. I always leave the store with a ward or two. I've looked at diagrams for effective use of CV but don't really have them committed to memory and doubt that at my level people follow the prescribed jungling routines anyway.
So to come (finally) to the point again, what should I work on to increase my usefulness to the team beyond the basics?