Here's what I've learned from friends of mine who race professionaly (their results speak volumes and I would really consider their advice incredibly valuable) This is, in general, what I've learned from them:

1. Most people who don't know what they're talking about don't like lifting weights because it temporarily slows them down or makes them feel slower. Results are not immediate.
2. Lift in the off season, while you're doing base training.
3. Do a lot of core (lower back, abdominals, obliques) training year-round.
4. Don't get a huge upper body, but don't neglect it. It's good to have some muscular balance for the sake of preventing injury, too.

Like I said, my opinion isn't worth sh*t, I'm a nobody with no results worth speaking of. I'm only passing on what I've learned from going to the gym with guys who's opinions are worth a lot.