It's not a "don't use" thing, as our youngest is not quite 7 months old, but have you looked at the "bumbo"?
It's designed to hold your baby in a sitting position before they can do it themselves - not sure about forcing development that way, but it's very convenient for keeping a little one upright and out of trouble.....
Reviews of them:
http://www.babyworld.co.uk/information/reviews/product.asp?id=137Good image of one:
http://www.bumbobabysitter.co.uk/We bought 2, one for the house and another for use in caravan and for me to use as a prop. I bought some zebra print, giraffe print and generic deer print material to use as covers for the ugly bumbo and parents can choose which material they prefer.
HTH,
Matt