If you know it's not a health issue keeping her awake, then follow the advice in "Healthy Sleep Habits, Happy Child" by Marc Weissbluth. He explains that often we parents train our children to stay awake or to have a hard time falling asleep on their own and at the right time, because we don't realize the importance of sleep or we worry our child will hate us or be traumatized. But that's not true. We followed Weissbluth's advice with both our children so far (we used his "rapid extinction" method), and while it is stressful to hear a kid screaming for a long long time, consistency makes a difference. It shows the child that you trust s/he can fall asleep on their own and that you'll all be just fine. It took our first son a week to sleep train, but he started sleeping 12 hours at night at four months. Our second son had colic and had a much harder time falling and staying asleep when he was a newborn, so it took a little longer for him to train, but he now (at 6 months) generally only wakes up once, at most twice, and is a much happier guy during the day because he gets more sleep. [I should point out that Weissbluth does not recommend trying to sleep-train until at least 4 months, at which point a baby is physiologically capable of sleeping at least 6 hours without eating. At six months, according to "Baby 411", an infant can sleep at least 12 hours without eating. Both our sons are healthy, happy, and intensely interested in learning, and I can assure you they have suffered no adverse effects from being left to cry it out.]
Weissbluth's goal is a well-rested family, not just a well-rested baby, and he does provide more methods and approaches than just letting the child cry it out ("rapid extinction"). He does point out, however, that rapid extinction is the simplest method to follow, and tends to have quicker results than more gradual methods. Everyone I've recommended this book to has also found it helpful, and I was given it in my turn by a friend.