Let's see...
Well, prior to a "fresh" game coming out, I always plan out who I want to use beforehand. There are a couple of things that I always do that hasn't changed since the original Red/Blue.
1) Start with the fire starter.
2) Get the starting bird
3) If the electric type in the region is not to my liking, trade over a pikachu/jolteon.
However, once Diamond/Pearl rolled around, I changed it to me Soft resetting for good natures/iv's on my starter (I wouldn't start the adventure until this was set in stone).
Months before HG/SS came out in the US, I actually prepared a team of pokemon in Platinum that I would send over (literally half of my team the first part of the game was traded over from Platinum). I breeded/Pal parked the pokemon that I knew I wanted to use, then ev trained them, so that once they were sent over to HG, I could enjoy the game like it was meant to "without" worrying about evs,ivs. I play to have fun first, but I also want to stand a fighting chance and have pokemon ready for wifi, so I take the little extra effort for the much bigger payoff.
Whenever a rare pokemon/legend shows up, I have my abra with the right nature in my party so that I can get the same nature and good stats.
Now that I've beaten HG, I'm going to prepare some different pokemon and then send them over to SS and repeat the process.