I go through getting Pokemon that I like. I don't build up a strategic team, focusing more on my favorite Pokemon. When I need someone for an HM, for example Fly, I'll think, "hmm, what flying type would I like the most?". When I see a new Pokemon I really like, I replace one in my team with it. I never really select my team before a game, though I sometimes do go back and pick and choose a team after I've beaten the game. It's actually worked pretty well for me, as I've battled my way through Fire Red, Soulsilver, Saphire, and Platinum, and I'm now working on White. And I never even tried to complete the Pokedex. I try to fill it up as much as I can, but I don't feel the need to complete it in its entirety. I just beat the story, then the post game, then I do some additional things after that, then I move on to the next game.