What is your champion select "System"?

I'm trying to figure out what to... do. Like, what to main, and how to go about it. If I'm going to support main, I can't just pick any support when it's needed that I want and have little experience with all supports. I'm trying to condense my champion selection down to just a few, and avoid 3 roles, but if I select support, I have to support always. I know if I do this, I can't always swap between Sona/Soraka/Lulu like I've been because sometimes a champion like Blitzcrank/Thresh or Leona/Alistar are needed/wanted. I guess I'm trying to ask, how does anyone maintain a constant flow of games with the same champion when other champions in that role are needed for that game? I really like playing with Alistar, but Leona seems to be a better pick. Same with Soraka, but Sona seems to offer more for the team. Any high elo/ranked players willing to offer their wisdom?