We have a strict, strict rule of only one activity at a time. (Another thing that helps in our household is that there is a 6 year space between our 2 kids, so I have an 8 year old involved in activities, and a 2 year old who is not. Also, we are not a big frenetic organized activity family, compared to most families we know. Our older daughter's first organized thing was a swim class on Saturday mornings, with me in the water with her, when she was 4 years old. The next thing was socccer when she turned 7.)
My older dd still does soccer. It's turning out to be way more time-consuming than I thought (practice 2 nights a week, and games on Saturdays), but she loves it so much and each season we have a conversation about, are you sure this is the one activity you want to do?
Soccer is in fall and spring. In the winter she might do some after-school activities that are on-site at her school (music, yoga, art - the parents at the school have organized a super-affordable after school program run by parent volunteers), so no transportation issues there. In the summer the organized activities we choose offer us an all-day option - like a day camp - and that's it - no evening activities except family ones - so there is no summer evening craziness.
Between DH and my flexible schedules, we manage to leave work early and rush her to her evening soccer practices, 2 year old in tow, dinner in a basket. Never easy! Whoever is on duty hangs out on the sidelines and entertains the little one, and we get home at the gruesome hour of 7. (I am never sure how we'll do it in 4 years or so when my youngest is 6 and beginning to do an activity!)
The other thing, as another poster mentioned, is to carpool wherever possible. I always take a sheet and circulate it amongst my daughter's team parents, asking if people want to share their email addresses and/or phone numbers and addresses, and then to indicate an interest in carpooling. I find the other parents and coaches are grateful for this, as I offer to make copies and circulate it, and set up a phone tree at the same time.
Finally, one more word: Subway! I use a Subway card and we get sandwiches about once a week. Every 3 weeks or so we end up getting a free sandwich with our "points." No, I have no financial interest in Subway.

I just find it to be a working mama's friend on those hectic nights when we are on the go!