I agree with the person who posted that her child was forgetful in other ways besides taking care of hygiene -- Think hard: Is your daughter forgetful about other aspects of her routines as well? Does she tend to forget to take her homework to school, forget to brush her teeth unless you hound her about it, forget to do other things that an 11-year-old should be able to remember on her own? If so, the toilet issues may be part of a larger problem and she may need lists like someone suggested. And 11 is not too old for you to set up a reward and discipline system for those lists: If she has really, really wanted something, like an outing to someplace, make that the reward for X days or a week of remembering to do things (including wipe). It's going to be tough to oversee this one, though -- you cannot be there to watch her use the toilet every time. I'm not sure how to enforce it.
Another route might be to give her a good book about her own body. American Girl publishes "The Body Book for Girls" which is excellent. It explains all the basics and includes hygiene. Though I don't think it ever directly addresses the idea that "You have to wipe your bottom or you'll get infections," the general tone is one of keeping clean and neat and feeling good about your body. You could use it to reinforce that idea with her.
Does she use deodorant? If she does and would never dream of smelling from sweat, you may be able to use that as a way to start talking about how hygiene "down there" is just as important -- if she values using deodorant under her arms, she should try to value keeping clean everywhere else.
It is a bit surprising to me that a girl of her age -- my daughter is 11 too -- isn't into being super-clean. This is an age when a lot of girls are so self-conscious about how they may smell, how they look, etc. But it's also an age where they can still get distracted and just want to move on to the next thing they want to do -- leading to a lot of "I forgot."