Koshi himo - soft ties for dressing. You'll need 3 or 4 of these typically.
Datejime - wider belt for dressing. Comes in velcro and polyester/silk vareties. 1 or 2 needed.
Obi ita - board for keeping obi from wrinkling. 1 needed.
Obi makura - "pillow" needed to help most obi knots hold their shape. 1 needed.
Korin - elastic belt with clips. Useful for holding collars in place. Not essential but recommended.
Eri shin - plastic piece to help collar hold its shape. 1 needed.
Underclothes - these can either be actual slips made for kimono or just underclothes like a camisole. Needs to not show when kimono is worn, but necessary to keep valuable kimono clean.
Obijime and Obiage - these are the "finishing pieces" on your kimono. Obijime is the rope-like piece; obiage is the scarf-like one. These are necessary for most obi knots.