Until the age of 6 mos, a child does not have the ability to self soothe. We had the same problem with our daughter. She slept on my chest for the first 3 months, but when Daddy came home from Iraq, this became a problem. I felt responsible for her not being an independent sleeper. I contacted an organization and asked all my questions. Basically, until 6 months, there wasn't much we could do. We had to rock her to sleep and then put her down, or I'd have to let her fall asleep on my chest before she went into her crib. I'm not going to lie, it wasn't the "ideal" situation, but I was finally able to share my bed with only my husband. And at 6 months, it was easier to get her to put herself to sleep. Try to avoid late naps in the day so she is tired at bed time. Do the same routine EVERY night (bath, bottle/nursing, book, bed). This REALLY gives baby cues that bed time is approaching and will help your little one ease into the night. Also, dim the lights as you near bed time.
I've got another baby on the way, and I'm already reading up on how I can not start the habit of baby sleeping with us. Hang in there!