I believe I have the first picture of her sitting unassisted... well, I suppose it's considered sitting "tripod" or something (this picture is from July 28, so she's 5 months old). She does great sitting like that until something exciting catches her eye, which usually happens to be my camera, the clicker, or my phone, and then she throws her entire body to the side and face plants on the quilt. Don't yell at me, I usually manage to catch her before she face plants, but sometimes she's too quick.
She does quite well sitting up in the boppy pillow since that gives her a little support around her bum... and it catches her softly when she topples over.
The other thing that I'm using to help her learn to sit is this toy meant for a walker that my Aunt gave her. It's one of those dual-purpose toys that at the younger age they sit in front of and push the triangle through the triangle hole, the square through the square hole, and so on and so forth. Then when they get older, they walk behind it. Anyway, I set her in the boppy and place this toy in front of her and she kind of hangs on to it while I put the triangle in the triangle hole, etc.
I never got one of those Bumbos (which are all the rave now)... I couldn't justify spending $40 on something that she would outgrow way too soon --yes I know that is pretty much the definition of a baby toy and clothes... something they grow out of way too soon -- but the Bumbo was just not on the top of our list. Besides, since I'm home with her during the day, I get to sit with her and play with her and teach her how to sit up myself. I think we're doing pretty well.