one of the moms on DD's soccer team has been helping us out all season by basically shuttling DD back and forth to practice (practices start at 4:00)

We literally wouldn't have been able to keep DD in soccer without her generosity.  I'd love to get her something really nice to show her how much we appreciate it but I'm kind of at a loss. 

I don't know her very well as a person so I'm hesitant to do food etc.

Maybe add in a nice bouquet or potted plant to go with a gift certificate for a spa like allinyc suggested, or a bookstore?  It's hard not knowing her too well.  Maybe a game night/family-centered basket - popcorn, board game, cards, other goodies?

I think everyones ideas so far sound great! When I am in doubt about a gift, I usually go with a nice thank you card with a gift card that allows them some choices - like one good at several restaurants, or a salon/spa type place.

I have seen too many parents take other parents for granted when helping out with the kids sports - It is very thoughtful of you to do something for her, and I'm sure she will appreciate whatever you decide.