First, take a look at her nap schedule. Perhaps she's just not tired when you want to put her down. Babies this age need about 14 hours of sleep total, about 10 at night and four during daytime naps. If your baby's started sleeping longer at night, her nap times may need to be adjusted accordingly.
When it is time for baby's nap, the one thing you shouldn't do is leave her to cry for 15 minutes and then rock her to sleep. This teaches her that if she cries long enough, you'll pick her up. You have two options: Either let her fuss until she learns to fall asleep on her own (while you frequently check in on her), or promptly pick her up after she starts fussing, play with her for five to 10 minutes, then put her down again. Remember, your goal is to teach your baby to fall asleep on her own -- how you get there doesn't matter.
Copyright© 2004. Reprinted with permission from the February 2002 issue of Parents magazine. Updated 2009