First off, don't worry if your DD isn't yet. I don't think the majority of babies are sitting up unassisted exactly at 6 months.
However, the best thing to do is give her a lot of time with a Boppy around her. My baby was sitting up relatively well at 6 months, but would fall over very easily. It wasn't until she moved to the next room at daycare (at about 6 1/2 months) where she spent so much time with the Boppy around her (instead of in the bouncer or on her tummy) that she was sitting up unassisted within a week.
How is her neck control? If it's not that good, continue tummy time. Is she rolling? This makes sitting up easier. Also, try to lay her on her back, hold her hands, and "help" (but really, just support) pulling her up to sit. This strengthens the abdominals. Neck, back and abs are important to hold her up in sitting.
Then as she is sitting, the boppy around her is a good idea. Or try and have her reach for objects while she is sitting up: up, in front of her, sidewards etc.
DD went from not being able to sit up at all one day to sitting like a pro the next. I swear the moment we just let it go and stopped trying to show her, she just did it on her own. Oh and she was 6.5 months when she started being able to. Your DD is doing fine.
Re: XP: How do you work with LO on sitting up?
First off, don't worry if your DD isn't yet. I don't think the majority of babies are sitting up unassisted exactly at 6 months.
However, the best thing to do is give her a lot of time with a Boppy around her. My baby was sitting up relatively well at 6 months, but would fall over very easily. It wasn't until she moved to the next room at daycare (at about 6 1/2 months) where she spent so much time with the Boppy around her (instead of in the bouncer or on her tummy) that she was sitting up unassisted within a week.
Agree with pp, don't worry.
How is her neck control? If it's not that good, continue tummy time. Is she rolling? This makes sitting up easier. Also, try to lay her on her back, hold her hands, and "help" (but really, just support) pulling her up to sit. This strengthens the abdominals. Neck, back and abs are important to hold her up in sitting.
Then as she is sitting, the boppy around her is a good idea. Or try and have her reach for objects while she is sitting up: up, in front of her, sidewards etc.
HTH.