Usually my first run consists of grabing and trying out team members as I go. There is however one rule I follow, if they are obtainable, my team must have a Skarmory and Flygon, cause there my sig pokes and almost on all of my in-game teams, i love those guys XD.
I'm going to only try out/possibly add the Pokemon I really like design-wise, focusing on using my favorites instead of obsessing with assembling the most competitive team. I'm also going to prioritize the new Pokemon over the old ones. Beyond that, I'm not going to plan a team ahead of time.