i have written alot about how to make baby sit from 6months onwards,,,let me see if i can find the post..i rem it well it was niddi and saavya kids who learnt sitting straight with all i wrote to them,,,let me see,,
here you go :

actual case started with this post:
http://www.indiaparenting.com/boards/showmessage.cgi?messageid=772&table_name=dis_milestones

followup of the above post:
http://www.indiaparenting.com/boards/showmessage.cgi?messageid=783&table_name=dis_milestones

rishabh starting sitting unsupportedly,,final followup :

http://www.indiaparenting.com/boards/showmessage.cgi?messageid=809&table_name=dis_milestones

8months baby not sitting, safe or not :
http://www.indiaparenting.com/boards/showmessage.cgi?messageid=788&table_name=dis_milestones

anu, what your baby is doing is perfectly normal,,she is also initiatng her stage to crawl...just make sure she does not hurt herself.its quite normal...and whenever she sits,,place your hand on back of his back and give a gentle but firm pressure,,she will learn to sit straight in longer run when you keep doing that !

Amma, at 13months,,most kids who might have learnt sitting before thr body backbone can support,,often sit with curved back. but its not too later for you to try somethings,,,
i would go by what your dr said but i want you to follow this :
1. there are firm pillow available in market,, and i strongly recmnd you to keep it behind her back whenever she sits on highchair,,since those pillows/back seats are firm though soft,,in order to make herself comfortable, she will have to sit straight !! practice that often....

2. whenever she is sitting and playing, and whenever she bends her back or if constantly her back is curved,,can you try giving a gentle pressure on his back and see if she straigtens herself,,,or check on her expressions...if she happens to feel uncomfortable, doesnt feel comfortable at all, i strongly recmnd you to make it a concern to your dr..he might suggest you some back pad that you can make her wear,,eventually it will help just the firm pillow i talked about in point no 1 and it should help..
because if your baby had ne other probs of backbone, by now she would have suffrrd from other milestone probs,,which is not the case,,its just that she got into this habit and you could not get a chance to correct it...so now start working on her.
also whenever she sits on her own chair or high chair,,make sure to keep the high chair straight and not elevated or on slant position....

3. make it a playtime and ask her to lay on her tummy.,and give her a gentle but firm massage with ne good lubircation(could be nething,,if nothing just some creme,,) or do it during bath or after bath,,she will like it..your baby is toddler she might not like sleeping on stomach for massage session,,so do it whenever you get a chance,,,continue doing that...
rest all i can tell you,,
amma there is nothin to worry,,soon your toddler will be 15months and if she is able to understand what you are saying,,start talking to her and make her understand that sitting straight is fun ! and that she can do it,,,see if preparing her mentally for it,,helps....often kids above 1year are good listeners,,but only thing we have to do is make it a fun time if we want them to listen to our commands,,see if it helps that way!

i hope this suffcs your queries...take care,,good luck

is your baby not at all aitting or sitting with a curved back? where is it curved? my daughter is now 7 months and she wanted to sit from 4 months. the ped checked her back and told she is normal. i am now making her sit propped with pillows and she is trying to sit. but the thing is she is always falling to teh front she wants to graba whatever that is in the front and she pushes forward and so her back looks really curved. i don't know i am just praying that ther should not be any problem.