I've avoided a lot of this because my 11 yr old son is very self-directed and not that into joining stuff. We'll see where my daughter ends up--too early to tell. But even with our very low-key schedule, this is what I've learned from my own experience and friends:
1)Set up carpools and don't set up an expectation that you'll be there for every game, event or whatever etc. Especially after age 10 I think it's a good time to shift to the idea that this is "their" thing and they do it because they love it and they don't need you cheering on for every moment. A lot of psychological research suggests this is pretty important for intrinsic motivation. Plus it's a good way to weed out real passions from the chaff.
2) Likewise, make participation in activities dependent on kids doing their homework/chores etc. without hassle. Unless you are the one insisting on the activity (i.e., music lessons) this is another way to see and encourage commitment and make your life easier. If baseball (or scouts, or whatever) is really a passion, they will work to get the stuff done. If not, you're one activity down and closer to a manageable family life.
just my 2c.