It sounds like she's forgetting who is the boss of whom and, although it isn't fun, you're going to need to remind her.
Is she older than you? My first job out of college my secretary was a woman in her mid thirties who had been there awhile and, meanwhile, I was 23. She would forget to do things I needed her to, take long lunches, spend her time gossiping with other secretaries. And I think it was because she felt she didn't have to respect someone younger than she was.
I decided to take her out to lunch one day and (nicely) explain to her that we were a team and I needed certain things from her. I told her that maybe she was used to doing things a certain way, but I had my way of doing things and that was how it was going to be. It was all very civilized and not unpleasant.
After that, I did have to remind her a few times but within a few weeks things were running smoothly. It seems that, with some people, if you don't sit them down and tell them directly that YOU ARE IN CHARGE, they will assume that THEY are in charge and can do what they want.
It's not always easy to do, but you really need to assert yourself. Good luck to you!
