My LO used to do this. Rather than wait until she was fast asleep i would let her feed and fall asleep and then slide my finger in her mouth and take her off. At first she would stir and then want to get back on. I would let her feed (this was generally short) and then try again. At the start, she might go on & off many times and then i would just let her stay on if it didn't feel like she would settle (and i was getting tired). But after awhile, it was less and less times that she would wake up again and try to attach. I think i got good at working out the best time to take her off and she got used to sleeping without nipple in her mouth.

HTH