What are some Best Camera Settings for Recording for Chroma Keying and which color of cloth reflects least green color?
Lighting is the biggest factor in improving this setup.  Move the subject further away from the sheet, use a lens that has a shallow depth of field so that any imperfections in the sheet are harder to see.  Light the sheet and the subject using diffuse lighting - large soft lights, or bright lights shone through large soft sheets, to make as few shadows as possible.  Backlight the subject slightly to get separation from the green screen and to reduce spill.

Camera-wise, use a camera which records as much colour depth as possible, and doesn't subsample the colour.  (Don't know how the Nikon samples colour)

If you're using After Effects, this is a very useful tutorial to get a good clean key, but without doing the steps above, you may still have difficulty getting a good key.