C.P.
When you catch her picking her nose explain to her that germs live in our boogers and we don't want to be sick. Have her get a tissue and put the snot in the tissue and then throw it in the trash. Kids love to be praised and told that they are doing a great job. Keep reminding her how grown up she is and how she is HELPING everyone not get sick. (I work on a schoolbus and I have watched a junior high boy do this and I almost puked!)