I make a mental list of the Pokemon I may want to use, but I don't check the strengths or IV's or any of that until I actually test them out and see how they're like. I also keep in mind the types because different types are always good. I also try to pick one or two that I had on my team in the past games that I know are pretty good.
For Pokemon X, in the beginning of the game I chose ones like Pancham and Skiddish that just looked really cute, but I ditched them pretty early because they weren't that strong, plus I needed room for Lucario and Sylveon.
A method for Pokemon X&Y that some people are doing is in the beginning of the game, when you get your starter and 5 pokeballs, you catch random pokemon and send your starter and the 5 newly caught Pokemon through Wonder Trade, and use those 6 new Pokemon that you got in WonderTrade throughout the entire game in your team. You can't add or subtract any from your team, just use your 6 Wonder Trades, no matter how terrible they are.
The WonderTrade method sounds possibly hard, but I really want to try it if I get Pokemon Y!
Edited by tinkeepie11, 08 November 2013 - 04:41 PM.