I've mentioned this in a couple of posts on other boards recently. I think the best piece of advice I got was to make sure *my* best friend was also my daughter's friend. So if there's ever anything they can't talk to you about then they have someone they can talk to that you know you can trust. We sort of combined that idea with the "godparent" idea. All of our girls have their own set of godparents that are all very close friends of ours. Having those relationships in our lives has been a huge blessing for all of us through the years.
I think the advice that was most helpful in the early years though (and I guess still is now) came from my mom. She told me to never think that I have to or even can do it all. She always said "Contrary to popular belief, mom's are not superheros." Accept help whenever it's offered and ask for it whenever it's needed. Even for the small stuff. I still strugge with actually using that advice but I try.
*She'd be upset with me if I didn't also say that we should offer/give help to others as much or more than we recieve it. That part I'm better at following I hope.