Hi C.,
I am a Mom of three - ages 8, 7 and 3 1/2. I had great skill with the sleeping thing after reading a whole lot. My youngest will be 4 in a month and still naps when he needs it (or I say he needs it).
It seems consistent that changing sleep habits takes three nights of steel nerves and maybe three more of some strength. So make sure you and your husband are mentally up to the challenge before you start. There can't be any emotion on your part - or conversation. Once the child goes down, they are supposed to stay there (ha ha :). You have to be stern - they recognise the voice. and after you get them to stay in the bed, they will cry and throw a fit. Mine went for an hour at one stretch before they got so exhausted they finally fell asleep. Unless you think there is a medical problem, let them go. Three nights of hell will pass and your daily misery will be over. Well worth the trouble.
Go to the library and find some books. You only need a chapter or two from each. Try Healthy Sleep Habits, Happy Baby, and Babywise or any others that look interesting!