When we had this problem, our pediatrician suggested that we let her cry it out. We would let her cry for 20 minutes, and then go in to check on her, pat her to let her know that we were still there, and then leave. We wouldn't say anything, just smile (if you can), and try again. She only did that for a few nights and then she fell asleep on her own. It's harder - much harder - for us to listen to her cry. It sounds hard, but it really helps when they learn to go to sleep on their own. She's been sleeping soundly ever since - for 6 years!!
The pediatrician suggested that I go take a shower while my husband did the checking and patting for a few nights... it helps!! And turn down the monitor!! Good luck! She was my first, and I am the mother of 3! ALL great sleepers!!