Interviewing

Hi, I'm looking for advice in regards to interviewing (professional careers). I'm a nurse who has been with a company for 1.5 years. Last year, I told my manager I was looking for other jobs because it wasn't a good fit. I never really found anything worth leaving for, so I stayed. I'm interested in looking again but I'm curious if I need to tell my manager that I'm looking once again. I wonder because I'm sure that they'll be wanting to contact references, and I don't know if I should warn her in advance? Do people normally tell managers they're in the process of looking? Or should I wait until I already get hired?