Hmm, if she wants it kind of focused on Elvis (I'm guessing the earlier years

), you might want to rent a movie or two of his from that era as well as others that would have bedrooms shown.
Another option would be to find wallpaper, fabric or a bedspread that you and your DD think would be a perfect starting point and use that for your colors. Here's what I'm picturing:
Bedding - kind of frilly but not over the top. Maybe ruffled layers around the bed and flat/quilted on top. Some throw pillows, fairly tailored in general but "retro" shapes like a round one with pleats that meet at the center circle (large covered button, maybe), square with a box shape - basically a flat piece of foam with cord trim around the edges and maybe some covered buttons used to "tuft" it. I'm also remembering some really weird looking stuffed poodles and cats. Do you remember the cat Shirley loved on the show Laverne and Shirley?
Walls - posters, 45 records - in or out of their covers, scattered around the walls. The posters would, of course, be mostly Elvis but some other 50's teen hearthrobs or movie posters would work as well (Marilyn Monroe, James Dean, etc.) For fun you could also include a large letter for your DD's first name.
Floor - something plush like fake fur for throw rugs.
Other - lots of tchotchkes (ceramic poodle and cat figurines) doilies and anthing else you can find at second hand or antique stores that would fit. I think it would be a lot of fun to make a fake "Diary" to sit on the nightstand next to DD's bed.
Colors - I'm thinking pink and turquoise with some accents in white and black (or silver).