i tend to feel that babies need help learning how to sleep - they have to learn how to latch on properly, then how to eat solids, wave/clap/crawl/walk/talk and all those other developmental things too. i read somewhere that very young babies 'forget' how to fall asleep - hence when they are so tired, but screaming - they've forgotten how to drop off, is the theory.
i read ferber's 'solve your child's sleep problems' after DD starting waking 3 times a night around 6 months, and i also started loosely following the gina ford schedule for day sleeps around this time (previously DD had day sleeps if she fell asleep randomly).
it took a few days to settle in, but everything fell in to place with eating/sleeping once i had a plan of action. DD needs her routine, she wanted to sleep (is my belief) but did not know how to. I was trying to be all casual, but my feeling is she got into bad habits and needed to be set on the right course.
at 9 months, DD now sleeps 12 hours 7pm-7am, apart from occasional night terrors, where me or DH will have to go in and sing to her, hold her hand, be physically there with her till she settles down again. usually about 2 hours...