A.B.
My cousin's daughter was doing the same thing, and she came up with a pretty good way to motivate her to stop. First my cousin made a chart with 7 boxes and gave her daughter a sticker for every day that she did not pick her nose. If all the boxes were filled with stickers at the end of the week, her daughter got to have a special little toy that she wanted as her reward for not picking her nose. But if she did pick her nose again, the toy would be taken away. This worked great for my cousin. Hope this helps!