I've been trhough PPD twice now, and my only advice is to make sure you have someone you can confide in. Choose someone who will listen, but not judge, and not be afraid of anything you may say - for example, saying you want to walk out of the house and leave your children behind, or that you feel you are a bad mother and the kids would be better off without you - very common things with PPD - but if you tell this to your mother or husband, they may not understand that it is an overwhelming feeling - they will either worry, or try to talk you out of your feelings.
But if your dh is someone who can be supportive without trying to fix things, and will not hang anything over your head, than he'd be a good choice. If not, reach out to another mother out there, your haridresser, a clergy member, or therapist even. You do need social contact of some kind, even if it is the last thing you think you want.
Also, have the phone handy for when you do feel down - books, movies, or visitors can all help lighten your mood as well.
In most cases, it will pass quickly - but feels overwhelming while going through it. The more support you have, the better.
If it does feel more serious - see a doctor -
I've read that PPD is stronger and more common in subsequent pregnancies. And it does not nescessarily have to occur immediately following birth - it can happen even months later.