Sew two inch pieces of velcro to the each end of a piece of elastic that is a about 2" shorter than the waist of the doll so that the elastic will be smooth when wrapped around the doll. Then position the doll in the chair or against something that you can put the velcro/elastic around such as the uprights on the chair. Put the elastic under the dress around the waist and the upright on the chair both. Fluff the dress over it. Doll stays put and you can reposition or whatever. If you have to go around something bulkier than the upright bar on the back of a chair simple make the velcro/elastic longer to accomodate the larger area. If the velcro/elastic will be seen, cover with a complimentary or coordinating fabric by making a tube and just sliding it over the the elastic, pulling it to cover after postioning.