C.S.
Hi C.. I struggle with this myself and find that writing up a schedule does help-- even if I can't follow it religiously.
Two other things that have helped huge around here:
1) Insist on the kids picking up their own toys. When straightening up toys 3X a day got off my to do list, suddenly I had so much extra time.
2) Delegate! Your older children and especially your husband can take some of the weight off. Let him take responsibility for certain things, or bathe the kids several times a week to free you up. If he complains, point out that if you guys can get the chores done faster together, there will be more time for the two of you in the evening, plus you won't be so tired (hint hint) ;)
3) I remember the exhaustion of having a newborn in the house, but I can only imagine what you're going through with 3 other kids, a husband and a house! If you can afford it get a maid to do the cleaning once a week or every 2 weeks. Also, sleep with your babe a lot and bf in bed. I remember when I needed a nap, just laying down to nurse and the baby would fall asleep right with me. Ahh, bliss!
Good luck! And congrats on the new babe.