Thanks for the suggestions, everyone. I'm running Goodstuffs myself, and I've definitely know my team's weaknesses, so I'll keep the suggestions in mind.
EDIT: Okay, maybe I jumped the gun. As said before, I'm running Goodstuffs and have problems picking other Pokemon against other Goodstuffs teams. Here's what I currently have in mind with Team Preview:
1) Look for the main threats to my team that force me to "counterpick" certain Pokemon.
2) On a similar note, try to predict what my opponent will bring. I can usually nail a couple of Pokemon that they'll bring along.
3) Look for common leads. (e.g. Fake Out users, Tailwind Zapdos, Latios, Cresselia, Chandelure, etc. are all fairly common leads in my experience.)
4) Try to examine any weaknesses.
And then I get overwhelmed. :\ It's hard to explain here, but I feel that I pick sub-optimal leads sometimes, which gives my opponent a slight edge at the start. Should I prepare for the common leads, or try to prepare for the worst-case scenario leads, etc.
Sorry if this isn't so simple.