Help a 7 year old Downs Syndrome child to become verbal

[Carole]

I have a 7 year old foster child with Downs Syndrome.  she is mostly non-verbal, using words and sentences only occasionally or in moments of stress when she will ask 'Are you okay?'  'Mommy please' etc.  Now and then she will repeat an entire sentence that I use as in 'Good morning Princess.'  On such occasions her speech is perfectly clear, her words well formed.  The remainder of the time, she speaks in mostly gibberish to her dolls and to herself.  How can I help this child on a daily basis?  Can anyone help?